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What is Motor Oil Made Out Of?


Abstract representation of a molecule of motor oil

All modern motor oil is composed of a base oil, which comprises the great bulk of the oil; and additives that modify, enhance, or augment the characteristics of the base oil to make the finished product more suitable for use as an engine lubricant.

The term base oil refers to the semi-finished motor oil before the additives are mixed in. The term base stock refers to the source of the material the base oil was made from.

The base oils of what we now call "conventional motor oils" use crude oil as their base stock. In fact, the earliest petroleum-based lubricants contained little other than barely-refined crude oil.

"Synthetic oils" historically referred to lubricants made from laboratory-synthesized hydrocarbons or other chemically-engineered materials that came from base stock other than crude oil. Nowadays, many motor oils marketed as "synthetic" actually use base oils derived from petroleum base stock, but which are much more highly-refined than base oils used in motor oils that are marketed as conventional.

Finally, some oils, most notably Pennzoil Platinum and Pennzoil Ultra Platinum, begin their lives as natural gas. These oils are classified by API as Group III oils and are marketed as synthetics, as are oils made from petroleum refined using severe hydrocracking.

Let's begin our study of the composition of motor oil by looking at base stocks, which are the raw materials from which base oils are made.


Motor Oil Base Stocks


Oil drilling rig in an oil field at sunset

Petroleum has been used as a lubricant for more than two thousand years, so it's not surprising that the first "modern" motor oils were formulated from petroleum. It was (at the time) cheap, plentiful and inherently slippery. But it was far from perfect.

One of the problems was that the composition of crude oil varies widely depending on the geology and natural history of a given area. Because early petroleum-based lubricants were minimally refined, the characteristics and quality of the finished oils were inconsistent at best.

Another problem was that there were no industry standards for motor oils anyway, and therefore no yardsticks against which quality could be judged. Anyone could say that their oil was the best. With no standards by which to judge an oil, anyone could claim anything.

It wasn't until 1933 that the Society of Automotive Engineers (now SAE International) came to the rescue by forming the Fuels and Lubricants Meetings Committee. They immediately took up the task of setting standards for motor oils, beginning with defining viscosity as the primary classification characteristic. That's why the letters SAE appear before the viscosity numbers of motor oils (for example, SAE 30 or SAE 10W-40). SAE invented and still administers the viscosity rating system.

The American Petroleum Institute (API), and later, the European Automobile Manufacturers Association (ACEA), also contributed by defining quality standards for a staggering number of motor oil characteristics; and another organization, ASTM International, defined standardized tests that could be used to determine whether or not oils met those requirements.

The existence of standards served as blueprints for advancements in petroleum science that have enabled petroleum-based oils to keep cars running smoothly for the better part of a hundred years. High-quality conventional motor oils like Valvoline NextGen, STP Pro Formula, and AmazonBasics Conventional Motor Oil are all excellent choices for drivers who prefer conventional oil.

Conventional oils are also considered by many mechanics to be the oils of choice for older engines with more than 75,000 miles (120,700 km) behind them, unless synthetic oil was specified by the vehicle's manufacturer. Older engines that have started to develop age-related problems like minor oil leaks or low compression may also benefit from a high-mileage conventional oil like Castrol GTX High Mileage or Pennzoil High Mileage.


Chemically-Engineered Base Stocks

Flask and dropper being held by gloved hands with periodic table of the elements in the background

Synthetic motor oils made from chemically-engineered base stocks are not a new invention. The superior lubricating qualities, better temperature stability, and lower volatility of synthetic lubricants were well-known as far back as the 1930's.

Early synthetic oils were much more expensive than conventional oils, however; and in older automobile engines with their simpler designs, looser internal tolerances, and absence of emissions-control systems, synthetic oils offered few advantages to justify their higher cost.

Beginning in the 1970's, automobile engines began becoming much more complex. The dual goals of reducing emissions and increasing fuel efficiency led to dramatic changes in engine design that put a great deal more stress on the engines. These advancements created the need for durable, lower-viscosity, high-quality lubricants that could handle the demands of lubricating smaller, harder-working engines with much tighter internal tolerances.

In 1974, Mobil 1 Full-Synthetic Oil was released to market and became the first full-synthetic oil to achieve mass-market success. It used a base stock of polyalphaolefins (PAO), which are synthetic hydrocarbons whose molecular structure and characteristics make them ideal for use as engine lubricants. Other popular synthetic base stocks include polyalkylene glycol (PAG) and various diesters and polyol esters.


Natural Gas

Four cooking range burners with natural gas flames

Like petroleum, natural gas is a naturally-occurring hydrocarbon, commonly known as a "fossil fuel". It tends to be more consistent in composition and generally contains fewer impurities than petroleum, however, both of which properties make it attractive as a motor oil base stock.

Early versions of gas-to-liquid (GTL) technology date back to the Second World War. As long as petroleum was cheap and abundant, however, the idea of making motor oil from natural gas on a commercial basis remained just that -- an idea. It just didn't make economic sense to invest the huge amount of money necessary to develop and perfect the technology to convert gas to a liquid motor oil on a commercial scale.

The oil crisis of the 1970's and the continuing instability in many petroleum-producing nations changed that. One company in particular, Royal Dutch Shell, spent about 40 years perfecting an economically-viable GTL process. What Shell came up with is literally the reverse of the fractional distillation and refining process used for crude oils.

Unlike petroleum refining, which breaks down big hydrocarbon molecules into small ones, Shell's process breaks down the small hydrocarbon molecules in natural gas and reassembles them into big molecules. The end result is an extremely-pure (better than 99 percent), crystal-clear crude-oil substitute that can be used to make motor oil and other products traditionally made from petroleum.

Because Shell's process actually disassembles the molecules in the natural gas, and then reassembles them into a new product, the resulting base stock is considered synthetic. It's what Shell has been using to produce their excellent Pennzoil Platinum and Pennzoil Ultra Platinum full-synthetic motor oils since late 2014.

The Elusive Definition of Synthetic Motor Oil

Synthetic oils are the lubricants of choice for most new car engines. Some manufacturers even require that synthetic oils be used, either by name or by specifying requirements that can't easily be met by conventional oils. Most car mechanics also recommend synthetic oils for newer cars. Even manufacturers of small engines for machines like snow blowers and lawn mowers are starting to recommend synthetics.

The problem is that neither the SAE, the API, the ACEA, nor ASTM International have come up with a legally-binding definition of what makes a base oil synthetic. Whatever tribologists may have to say about the issue, on the consumer level, "synthetic" is just a marketing term. Consequently, although few would argue that synthetic oils have many advantages over conventional oils, we don't really have a clear-cut definition of what a synthetic oil is.

Base Oil Groups

What we do have, thanks to API, are "base oil groups." The API defines five base oil groups by the base stocks from which the base oils were derived, the manufacturing process used, the percentage of saturates (basically, very tightly-bonded molecules that make the oil more resistant to breakdown) in the base oil, the sulfur level, and the viscosity index. Here are the main characteristics of the base oils in each group.

Group I Oils

Group II Oils

Group III Oils

Group IV Oils

Group V Oils

All oils in Groups I and II are classified as "conventional" oils.

Oils in group III, although derived from crude oil or natural gas, are marketed as synthetic oils in most countries. You can read why here. Most tribologists also consider them synthetic because of the extent to which the molecules in the base stock were disassembled and reassembled into a new substance.

All oils in Groups IV and V are synthetic by anyone's definition.


Are Synthetic Motor Oils Really Better?

Usually, yes. Good synthetic base oils have better wear protection, temperature stability, viscosity index, pour point, and extreme-temperature characteristics than conventional oils; and therefore require fewer additives to make them suitable for use as engine oils. They also tend to be less volatile and less prone to forming deposits, which is especially important if your car has a GDI engine (more about that here). They also may improve your fuel economy, are more resistant to breakdown, and don't need to be changed as frequently.

In short, even if your car's manufacturer doesn't specifically require a synthetic oil, it's hard to think of a good reason not to use a good synthetic in a new or newish car. In every way that matters, synthetics as a group outperform conventional oils.

That being said, it takes a lot of work to make a good synthetic oil, and all that work costs money. With motor oil, as with most products, you get what you pay for. The fact that a motor oil says "Synthetic" on the label doesn't mean that it's a good synthetic. Considering how trivial the cost of oil is as a percentage of the total cost of owning a vehicle, I advise you to stick with oil made by reputable manufacturers, and to aim higher than the bottom shelf.


When Shouldn't You Use a Synthetic Motor Oil?

As mentioned earlier, many mechanics do not recommend synthetic oils for older vehicles, especially if they have been using conventional oils for a long time.

Some people claim that differences in the flow characteristics of synthetic oils make them unsuitable for older engines because they "flow through the engine too easily" and will not properly coat the moving parts. Personally, I think that's hogwash. If the oil is of high quality and is of the correct viscosity, I think it will coat the parts just fine. But some people disagree.

One more likely problem, however, is that the superior detergency of synthetic oil may wash away the sludge that is, in effect, helping to seal an older car's engine. That does happen from time to time. Of course, a case could be made that using sludge to seal your engine is a less-than-wonderful idea anyway; but if all you want is to get another year out of your old beater until you can afford to buy a new car, maybe you don't want to push your luck.

If you would like to help your old car live a even longer life (and if it's started to leak oil from the seals), then you may want to consider a full-synthetic or semi-synthetic high-mileage motor oil. Synthetic oils made for high-mileage vehicles are often diester-based because diesters tend to swell seals. (PAO-based oils, on the other hand, tend to shrink seals unless additives are added to the base oil.) High-mileage oils made from base oils other than diesters usually have seal-conditioning additives that help prevent or stop oil leaks.

My personal advice is to not use high-mileage oils in a car that doesn't already leak. Swelling seals that don't need it may actually accelerate seal wear. But if your older car's engine does starts leaking oil, then a high-mileage synthetic may fix the problem for a while at little cost. (Replacing the seals would be even better, though.)

Finally, if your car's manufacturer specifically recommends against using synthetic oil, then you should not use it.


That concludes the discussion of base stocks. We'll talk a bit about some of the additive and modifiers that are the other parts of what oil is made of here.