All about motor oil (1980)

by Greg Raven

If you have been a regular reader of my magazine articles in VW&Porsche, European Car, VW Trends, and elsewhere, you know that I am somewhat fanatical on the topic of motor oil quality. Within the bounds of my limited resources, I have endeavored to bring to light the good, bad, and the ugly in the lubricant business. Some companies have praised us, some have sulked, and others have threatened legal action.

The most important thing to come of all this, however, is that many readers have followed my lead and investigated the differences for themselves, with almost entirely positive results. So I thought it might be valuable to take another in-depth look at what makes motor oil do what it does, to help you understand what makes one motor oil perform differently from another.

The Basics

Before you can evaluate the current crop of engine lubricants, you first must look at what demands the engine places on motor oil.

Allow Easy Starting

The very first thing oil must do is permit the engine to start easily. This sounds deceptively simple but, in fact, it requires that the oil is formulated carefully to allow the engine to turn without excessive resistance when cold without compromising lubrication when the engine becomes hot. Ideally, it would also form a protective coating on engine parts even after the engine is shut down. This coating could then provide intermediate lubrication as soon as you turn the ignition key. These first few seconds, before the oil pump has time to fill the oil galleys, is when the highest percentage of engine wear occurs. A little prevention goes a long way, but a coating that is too thick or gummy could prevent easy starting.

Prevent Wear

Once the engine is running, the oil must prevent metal-to-metal contact by establishing a complete and unbroken film of oil on all critical surfaces, what lubrication engineers call full-film lubrication. Just about any liquid can be called on to provide full-film lubrication under hydrodynamic conditions, such as exist between moving parts (think of tires hydroplaning on a wet street). It is what happens when the two pieces of metal stop moving (hydrostatic conditions) that interests us (such as when the piston reaches dead center, and the rings stop relative to the cylinder walls). Anytime there is metal-to-metal contact, such as during starting when it is almost impossible to maintain full-film lubrication, the situation is termed boundary lubrication. When this occurs, the friction generated between the surfaces can produce enough heat to temporarily fuse metals together. You might say boundary lubrication is a fact of death in engines.

Another good reason to stop this initial wear is that it then makes subsequent wear less of a problem. For example, even a small amount of primary wear will result in rough spots on the mating surfaces and metal particles in the lubricant. Rough spots wear faster than smooth metal, aided by the dislodged wear particles that are roaming around looking for trouble. This is called secondary wear, because it would not have occurred but for the primary wear. Eliminate the primary wear, and the secondary (and tertiary, etc.) wear problem ceases to exist.

Minimize Friction

When there is full-film lubrication between all metal parts, the internal friction of the oil becomes a factor in engine operation. If the oil is too thick, it will resist the efforts of the metal to move relative to the oil and create drag on the parts. If it is too thin, the metal will force its way through the oil film and create boundary lubrication. The thickness of the oil must, therefore, be balanced to provide the best compromise between these two extremes.

Protect Against Rust and Corrosion

Even in a perfectly sealed engine with pure fuel, water will get into the engine and condensate on engine parts. How? For every gallon of gasoline that is burned, a little over a gallon of water is formed as a by-product. Since the formation of this water vapor is part of normal engine operation, it is part of the oil’s job to prevent rust.

The engine is open to another form of corrosion, too. When the oil becomes contaminated with soot particles from the combustion process and is heated beyond a certain point, it turns from its normal alkaline state to an acidic state. This acid can attack the metal engine parts and cause corrosion unless neutralized quickly.

Keep Engine Parts Clean

In every internal combustion engine, there are contaminants developed as by-products of the combustion process. These are in addition to other contaminants such as dirt that gets past the air filter, sand and metal particles that come out of the block, etc. The oil filter is designed to catch the large contaminants, but containing the smaller particles is the job of oil additives that serve as collectors of — and storage areas for — anything too small for the oil filter to handle.

Burn Cleanly

The oil is called upon to lubricate such tricky areas as the cylinder walls and the valve guides and stems, which means that some oil is going to enter the combustion chamber and be burned along with the fuel. If it leaves a residue, piston rings can become stuck, dropping compression and overall engine efficiency. If the oil leaves metallic deposits when it burns, they can clog the electrode on the spark plug and cause misfiring, which will also drop engine efficiency and accelerate the formation of sludge materials. If the oil leaves behind a crystalline or abrasive residue when it burns, cylinder walls and bearing surfaces will be slowly but surely machined away as the sharp-edged crystals circulate through the motor.

Of course, a build-up of black carbon on the head of the piston and on the combustion chamber attracts and absorbs heat to a far greater extent than does a clean metal surface. This extra heat can cause detonation, place a higher demand on the engine’s cooling system, and hasten oil break-down.

Keep the Engine Cool

Because only about 60 percent of the engine cooling is handled by the radiator and coolant, the other 40 percent (more in an air-cooled engine) must be taken care of by the engine oil. The combustion process takes place at about 2,000 to 3,000 degrees F, which can heat pistons and valves to 1,000 degrees F in extreme cases. In pistons, much of this heat travels down the connecting rods and affects the bearings. Since tin and lead, two common bearing materials, soften drastically around 350 degrees F and melt at 450 degrees F and 620 degrees F (respectively), it is important for the oil to transfer excess heat away from the bearings as quickly as possible. In valves, the long, thin valve stem is more easily stretched when hot as the valve spring pulls the valve tight against the seat. Too much stretch and valve clearances disappear and valves and seats burn.

There are ways of helping the oil keep its cool without resorting to chemistry. Increasing sump capacity increases the length of time the oil gets to cool off before being thrust into the breech again, and the more oil there is the more BTUs it takes to heat it up and keep it hot. Adding an oil cooler allows the oil to more readily lose heat, and can add to the volume of oil in the engine.

Incidentally, installing a high-volume oil pump to cure high oil temperatures actually exacerbates the problem in most engines. A high-volume pump takes more horsepower to run (creating heat), and it over-pressurizes oil passages, which can lead to greater oil consumption as the oil is squirted or flung onto the cylinder walls or past seals.

Seal the Combustion Chamber

The piston rings and cylinder walls may look smooth to the naked eye, but they are microscopically rough. In order to better seal the combustion chamber during the compression and power strokes, a thin film of motor oil must fill in all the little discontinuities. Obviously, it does this in addition to lubricating the contact points between the rings and the cylinder walls.

Resist Foaming

As oil is splashed around the engine by the rapidly moving engine parts, foaming can occur. Unfortunately, foam is a poor lubricant and a poorer heat exchanger. Therefore, foaming must be kept to a minimum.

Be Part of the Solution, not Part of the Problem

A pressurized cooling system with a 50/50 water and coolant mixture does not begin to boil until 266 degrees F. In air-cooled motors, only the engine design determines what the operating temperature will be. Modern automobile engineers are exploring the limits of higher engine temperatures in their efforts for better fuel economy and lower pollution.

The drawback is that the rate of oil oxidation doubles for every 18 degrees F increase in temperature. Thus at 254 degrees F, the oil is oxidizing eight times faster than it would at 200 degrees F. When oil oxidizes, two things happen. First, smaller molecules glob together to form bigger, lazy molecules. This is the famous viscosity breakdown that you hear so much about. Oxidation is joined in its task by sludging, nitration, and polymerization to thicken oil. Second, the oil turns acidic, and this condition can cause corrosion for the very engine parts the oil is supposed to protect.

Factors Affecting Oil Performance

The way you drive has a big affect motor oil performance. Repeated cold starts can cause excessive fuel dilution. Short trips around town can lead to abnormal water accumulation. Unless the engine reaches its ideal operating temperature, these volatile contaminants are left to attack engine parts instead of being burned off by engine heat. Even after the engine has reached normal operating temperature, it takes a while for the engine to recover from the abuse of this type of driving.

Driving in hot climates and/or towing and/or other severe duty use of your car can also cause oil oxidation and sludge and deposit build-up. The ideal driving condition is high-speed freeway driving on paved roads in dust-free areas. If this describes the majority of your driving, you are virtually assured of good oil and engine performance. While you may not be able to change your driving habits, you can at least be aware of what is happening in your engine, and compensate for them to ensure a long life of your car.

Base Stocks

Now that you understand a little about what the oil is up against, let us look at what oil engineers have to work with. Most motorists are most familiar with mineral oil base stock lubricants, as they account for close to 98 percent of the motor oil sold in America today. This was not always the case, however, and the day may come when mineral oils again take the back seat.

Traditional base stocks

In spite of its dominance of modern lubrication, mineral oil was not always the lubricant of choice. Olive oil was used to lubricate wooden planks some 3500 years ago for moving heavy loads. Throughout history, oils made from rapeseed, castor beans, palms, lard, wool grease, and sperm whales have been used. Some of these materials are still in use today, combined with modern additives.

One motor oil, for example, combines a mineral oil base with a percentage of oil from the jojoba plant, along with an additive package. Another manufacturer makes a lubricant for racing cycles with a pure castor bean base. These so-called natural oils are different from mineral oils in that they have oxygen atoms in the molecule in addition to the hydrogen and carbon atoms.

Mineral oil base stocks

Mineral oil base stocks are refined from crude oil, the stuff they pump out of the ground. By varying the temperature and pressure at which the crude oil is processed, refiners can crack the crude oil to obtain asphalt or jet fuel, and everything in between.

Crude oils fall into two categories, paraffinic (as in wax) and naphthenic. Within each of these two categories are two sub-categories, neutral stock and bright stock. Paraffinic oils have a naturally high viscosity index (see below for an explanation of viscosity index), but are converted into varnish and hard deposits under heat. Naphthenic oils are naturally clean and are clean burning, but have poor viscosity indexes.

Whichever type of crude they start with, after cracking the refiners are left with lighter and heavier weights of base oil. Neutral stock is the thinner component of oil, while bright stock is the thicker component. When compounding petroleum oils, engineers are confronted with balancing the pros and cons of these four diverse elements in order to come up with a usable base stock.

Synthetic base stocks

Animal, vegetable, and mineral oils are either extracted or refined. But there is another way to obtain a lubricant, and that is to build it from scratch. This is how the synthetics are made.

Historically, synthetics have been around in one form or another for close to 60 years, starting with research into new hydrocarbon liquids in the laboratories of Standard Oil of Indiana in the early 1930s. During WWII, German scientists developed synthetic lubricants that would allow their machinery to operate in the intense cold of the Russian front. In the late 1940s, British and American scientists realized that the new jet engines would need a special lubricant if they were to fly at all, and synthetics were developed for that application.

But all synthetic base stocks are not alike, either in construction or in behavior. Out of the 18 or more synthetic base stocks, only four are currently of interest to us as internal combustion engine lubricants. Dibasic acid esters (diesters) and monobasic acid esters of polyol (polyol esters) are both of the ester family. Alkylated aromatics (dialkyl benzenes) and olefin oligomers (polyalphaolefin, or PAO) are of the synthesized hydrocarbon family.

Comparing the Base Stocks

As you might have gathered from the discussion thus far, there are several categories of performance that concern oil engineers. The accompanying table shows some typical considerations in judging base stocks. Each of these categories merits our consideration, as well.

Compatibility

Of course, all conventional motor oils are compatible with each other, but what about the synthetics? You would never mix a synthesized hydrocarbon oil with an ester oil without careful testing, but you might be forced to mix either of those with conventional motor oil in a pinch. If this is an important consideration, then the choice would be either of the synthesized hydrocarbons, like polyalphaolefin. The esters do not do as well in this category, and some of the early esters turn to jelly when mixed with mineral-based motor oil and a little condensation (fortunately, products these esters are no longer on the market). If in doubt, consult the manufacturer before you are forced to experiment on your own.

Viscosity Index

Not the same as the viscosity rating, the viscosity index describes how well an oil resists thinning at high temperatures and thickening at low temperatures. It is not so much an absolute rating as it is an indication of how the oil will perform relative to its own viscosity rating. A high viscosity index means the viscosity of the oil will undergo little change due to variances in temperature.

Pour Point

This is the measure of the oil’s ability to function when cold. The importance of a low pour point is a prime reason to consider synthetics when the temperature dips.

High Temperature and Oxidation Characteristics

The figures given in the table above are the unweighted averages of four different oxidation tests, the ASTM Rotary Bomb, the Modified Rotary Bomb, Cigre, and Beaker. As susceptible as conventional motor oil is to oxidation, synthetics are not inherently less susceptible. This would tend to indicate that an oil cooler is a good idea if your oil temperature runs much over 210 degrees F.

Volatility

Volatility is the measure of the amount of oil that boils off under heat. It is no surprise that our old pal the petroleum motor oil takes a beating on this one. These figures are derived from ASTM D-1160 tests. These tests validate the claim that chemically pure base stocks are better, as the distillation curves of the diesters are nearly flat.

Affect on Painted Surfaces

If your car is not painted with an epoxy, it has either an acrylic or lacquer finish, both of which can be susceptible to damage when in contact with oil. Even though you do not normally rub motor oil on your paint, some base stocks are less hazardous in this respect. In this case, again, conventional motor oils and the synthesized hydrocarbons are harmless, while the esters have a slight to moderate effect.

Seal Swelling

Most common seal materials are compounded to resist either swelling or harden ing in contact with conventional motor oils. A lot has been made about synthetics causing gasket material to go soft, creating leaks. There is some truth to this, as the esters, by their very composition, act as plasticizers that can and will soften plastic and some rubber products. One manufacturer claims to have licked this problem, but another with a similar ester base lubricant lists either plastic and rubber products that are not recommended for use in contact with their oil. These include neoprene, SBR rubber, Low Nitrile Buna N, polystyrene, PVC, and ABS. It should be pointed out that neoprene is a common gasket material. This is not to say that conventional motor oils are perfect, as anyone can testify after seeing the effect of motor oil leaked onto suspension bushings.

Stability in the Presence of Water

With one gallon of water created as a by-product of the combustion of one gallon of gasoline, you do not need a leak in your cooling system to get moisture in your oil. The oil must remain stable when mixed with water and retain its lubricating qualities after the water evaporates off. Esters are not as good as either conventional motor oils or synthesized hydrocarbons in this regard.

Rust Inhibition

Oil prevents rust, right? Not necessarily. At least, not all oils do it in the same degree. Conventional oils and synthesized hydrocarbons are better than esters in rust prevention. Under normal circumstances, this would not be a factor, but for vehicles that are stored, or for engines that never reach a sustained high temperature (making only short trips during the winter, for example) rust inhibition could become critical.

Additive Solubility

This is almost a trick category. Years of research into additives for conventional motor oils have nearly perfected a technology that ester oils cannot fully utilize. However, synthesized hydrocarbons are close enough chemically to take advantage of most conventional additive chemistry. For esters, this problem has been largely overcome in the last several years. For PAOs, lighter base stocks work best, as additives can drop out of suspension if the base stock is too heavy.

An example of the differences can be seen in a Caterpillar 1G test run on four oils including mineral oil, dialkyl benzene, polyolefin, and diester, all with the same additive package. Results varied wildly. The mineral oil failed the test after 120 hours due to excessive deposit build-up. The dialkylated benzene failed at 360 hours with, among other things, a stuck top piston ring. The polyalphaolefin went the full 480 hours with acceptable carbon formation. So did the diester, but the diester-lubricated engine showed extreme wear on the cam lobes and lifters. Clearly, for these four oils to be equal in performance, each would need a different additive package.

Synthetics vs. Conventional Oils

As you might guess from the example above, the differences between synthetics and conventional oils really begin to show up in actual usage tests, as opposed to strictly laboratory tests. That is why standardized engine tests were developed. Among these are what is known as the SAE Sequence IIIC test.

This rigorous trial is conducted in a blueprinted Oldsmobile engine run 64 hours at 3,000 rpm with a heavy load. The oil temperature in the sump is maintained at 300 degrees F. Oil samples are examined every eight hours, and afterward, the engine is torn apart and analyzed for wear. This test measures viscosity increase, piston skirt varnish, sludge development, ring sticking, lifter sticking, and scuffing and wear of the cam and lifter parts.

Another test, known as the Sequence VC, measures oil screen clogging, among other things. Test technique L-38 measures bearing metal loss by weighing the bearings before and then again after the test.

All good motor oils pass these tests with room to spare. The synthetics, however, often pass so easily that it seems almost pointless to test them. In fact, many synthetics have been tested at double the time called for in the original tests, and still, have passed with flying colors.

Who Uses What?

In the old days, oil engineers took much the same approach to compounding synthetic oils as they had taken with mineral oils; get the best base stock you can and compensate for its weaknesses with additives. Back then, of course, additive technology was not as advanced as it is now, and the insistence on sticking with a pure base stock lead to some disasters and near disasters.

Nowadays, pure synthetic base stocks are almost exclusively the domain of the diesters, as used by Amsoil and many others. The other synthetic base stocks are almost always blended with something else. This is the case with both Mobil 1 and Spectro Racing oils, which combine diester and PAO. Although this was not always thought to be the way to blend a super oil, oil engineers now know that the trick is to blend diester and PAO in the right proportions and with the right viscosities. Many companies also blend synthetic (diester or diester/PAO blend) with petroleum base stock, the goal being to compromise between the performance of synthetic and the cost of mineral oil.

Interestingly, the most synthetic sounding of the oils, Synthoil, is not a synthetic. Synthoil uses a highly refined mineral oil base stock (known as white oil), which is then fortified with a proprietary polymer additive. This results in a very hearty lubricant that avoids the compatibility issues that sometimes face true synthetics.

Typical Oil Additives

Responding to the needs of the modern motor, oil manufacturers have, to a greater or lesser degree, developed oil additives that help boost the performance of the base stock and offset the negative effects of engine operation. For example, straight mineral oil lubricants, no matter how well refined, are mighty poor performers in the engine compartment, and for that reason are almost never used without a whole slew of additives.

As necessary as these additives are, they do have their drawbacks. For example, not all additives are lubricants. They may help the base stock do its job, and may even be vital, but they are not lubricants themselves. If, however, you have a base stock with many of the characteristics of the additive package naturally, you can leave more oil in the final mix and still have a superior lubricant. In some oils, up to 20 percent of the volume is taken up by the additive package.

There are a great many additives, some working to correct more than one problem, but they fall into these general categories:

Pour-Point Depressants

Let us take straight mineral oil as an example again, and look at low-temperature performance. A typical mineral oil base stock will cease to flow or pour in the region of 10 to 25 degrees F. Although this may sound like a low enough temperature for many drivers in the warmer states, you must remember that the oil thickens gradually as it gets colder, so that even at moderate temperatures there might be enough thickening to prevent an engine from cranking fast enough to start easily. In order for these motor oils to be useful in cold climates, they need to flow at much lower temperatures than those at which they will actually be used. For this reason, pour-point depressants are added to the oil base stock. This additive works to restrict the growth of wax crystals in the paraffin base stock. These crystals form when the temperature drops, blocking oil flow. These depressants also aid oil circulation after cold engine starts.

Anti-wear Additives

Anti-wear additives are also called anti-scuff additives, friction modifiers, extreme pressure (E.P.) agents, film strength agents, and oiliness agents. They consist of oil-soluble compounds with a strong affinity for metal and supplement the lubricating qualities of the oil. Zinc dithiophosphate is one such additive, although there are many others, some of which will be discussed later. It is interesting to note that some anti-wear additives are temperature-sensitive. For a street oil, the anti-wear additives would turn on at a lower temperature than those in a racing oil.

Anti-wear additives, along with corrosion and rust inhibitors and oxidation inhibitors (see below), are weak acids. Their presence is indicated by the Total Acid Number (TAN). With TAN, the lower the number, the better. A typical new oil might have a TAN in the range of 2.5 to 3. When the TAN reaches the neighborhood of 10, it is time to change your oil.

Corrosion and Rust Inhibitors

Corrosion inhibitors actually do two things in the engine. They neutralize corrosion-causing compounds that enter from outside the engine and help fight the tendency of the oil itself to turn acidic.

Corrosion and rust inhibitors can work in either of two ways, depending upon which type they are. They can be oil-soluble chemicals, with a greater affinity for metal than water (forming a barrier), or they can be of the type that surrounds and insulates the individual water and acid molecules. Modern motor oil will contain one or more of these chemical inhibitors.

Oxidation Inhibitors

As petroleum oils oxidize they turn into either organic acids (which attack metals) or oxyhydrocarbons (which cause oil thickening and sludge). Neither by-product is desirable in the engine, hence the need for oxidation stabilizers. Surprisingly, in oxidation test run on motor oils, mineral oils bested synthesized hydrocarbons and esters in three of the four different tests. Relative oxidation stability varies, depending on the base stock, but from these tests, it can be seen that no single synthetic base oil has a distinct advantage in the oxidation department, and that synthetic base stocks are not necessarily less susceptible to oxidation than mineral oil base stocks. Anti-oxidants fall into two categories. The first type slows down the rate at which the oxidation takes place, and the second type forms a protective coating on bearing surfaces to shield sensitive metals.

Detergents and Dispersants

One of the first things you learn about engine lubrication is that there seems to be a conspiracy to turn your perfectly good motor oil into sludge. Sludge can loosely be defined as any insoluble material formed as a result either of deterioration of the oil or of contamination of the oil, or both. Straight mineral oil is defenseless against sludge formation. Detergents and dispersants are used in combination in modern motor oils, serving to prevent sludge accumulations in the engine.

Motor oil detergents are oil-soluble metals that operate in a manner similar to the water-soluble laundry detergents with which we are all familiar. Some of the more common detergents are barium, calcium, magnesium, and sodium. Neither detergents nor dispersants do any lubricating themselves. Therefore, the more detergents in the package, the less lubricant. Also, being metallic in nature, detergents form ash deposits when burned in the combustion chamber.

Dispersants are non-metallic, and are ashless when they burn, although they are quite a bit more expensive than metallic detergents. Where detergents turn on at higher temperatures, fighting varnish and neutralizing acid, dispersants operate at low temperatures. The measure of dispersant and detergent strength in new oil is called the Total Base Number (TBN). It relates the amount (the higher the better) of weak additive alkalinity in the oil. A typical oil might have a TBN in the range of 5.5 to 6, while a heavy-duty diesel oil might have a TBN in the range of 7.5 to 10, due to the fact that it has to counteract the acids of sulfur present in diesel fuel. There is more to oil that TBN, so a high TBN is not necessarily indicative of better motor oil. On the other hand, once the TBN reaches 1.5 the oil is pretty much shot.

Detergents and dispersants work by trapping floating dirt and sludge particles while they are still small. Although the dirt and sludge are in suspension, it has been demonstrated that oils with detergent and dispersants show remarkably less wear than similar motor oils without them. Any corrosive or abrasive material big enough to score the bearing parts should be caught in the oil (or air) filter. Since filtration is a whole ’nother story, let us assume that the filter is not the problem, and continue on with the oil itself.

Foam Inhibitors

Watery, frothy oil spells trouble for your engine. Foam inhibitors do just as their name implies; they weaken the surface tension of the air bubbles and cut oil froth. Chemically, a few parts per million of silicones (hydrogen and silicon compounds) are enough to reduce the foaming to acceptable levels.

Viscosity Index Improvers

What happens when you need an oil to flow easily on cold mornings, and also provide protection when the going gets hot? Your natural impulse might be to take an SAE 10W oil and an SAE 30 oil and mix them to get the oil you want (see the Classification sidebar for an explanation of SAE viscosity ratings). Unfortunately, what you would wind up with would be closer to an SAE 20 oil than to the SAE 10W-30 you needed, and it would satisfy neither of your requirements.

To make a true multi-weight oil, oil companies take an SAE 10W base stock and add Viscosity Index Improvers. V.I. Improvers are special polymeric compounds that reduce the tendency of oil to thin out at higher temperatures. Taking our SAE 10W-30 as an example, the V.I. Improver would have little effect at 0 degrees F, and the oil would test out as a straight SAE 10W oil. At 210 degrees F, the V.I. Improver would be working, with polymeric action, to make the SAE 10W oil behave like an SAE 30 oil. By selecting the base stock and the type and quantity of synthetic polymers to add as V.I. Improvers, different multi-graded oils can be made. The use of V.I. Improvers allows oils to have wider operating temperatures.

It is interesting to note that under high temperatures and hard use, the base stock gets thicker while the V.I. improver shears down (wears out). What you are left with is a thick oil with a very low viscosity index. This is why most oils need to be changed periodically no matter how good the filtration is.

Choosing an oil

Whether or not you ever develop an understanding of the basic components and manufacturing compromises that go into a quart of oil, it is not difficult to see that neither reading the list of ingredients (if you could get them) or taking the word of some highly-paid celebrity is going to be any guarantee that you are getting the best oil.

Unfortunately, testing the claims made about an oil can be involved and expensive. The Sequence IIID, Sequence VD, and L-38 oil tests cost thousands of dollars. Most oil manufacturers can afford this but most enthusiasts can not. In lieu of empirical data, frequent oil and filter changes (augmented, if possible, by a high-performance oil filtration system) using name brand oil and filters will go a long way towards protecting your investment in your car.

Unless you have some sort of hard evidence to back you up, you should always buy an SG or CD rated motor oil … at least, you should until the ratings are upgraded.

This is true even for cars that have been run on non-detergent oil their entire lives, in spite of the rumors that they will self-destruct if introduced willy-nilly to a detergent oil. The argument goes that the detergent oil will knock loose a bunch of sludge that will clog the motor, necessitating an overhaul.

Fact: the detergents are not strong enough to scour the inside of the engine, as myth would have it. Fact: an engine with pockets of sludge is living on borrowed time at best. Fact: testing has shown that less engine wear will result from holding the particles in suspension.

One last thing: Do not try to run your favorite SAE 10W30 oil in the middle of summer in the California desert, and do not try to run an SAE 30 in a Minnesota winter just because it is synthetic. Consult your owner’s manual for the manufacturer’s recommendations; they have done the testing, and are in the best position to know what will give you the best protection and fuel economy.