The Science of Oil

What IS your oil?

So obviously something happens to it between the time it gets pumped out of a well and the time it gets poured into your engine. I hadn't put a lot of thought into it before I took an machinery lubrication class for my work about 15-years ago.

Think of your oil as being two components... the base stock, and the additives. For grease there is a third component, the thickener, but this post is about oil, not grease. Let me address each component separately.

Base Stock: This is the oil part of the oil... that is to say the petroleum that we begin with to make motor oil. There are two general starting places for oil, mineral oils and synthetic oils. The mineral variety which is the original, is essentially a refined component of crude oil. API (the American Petroleum Institute) categorizes oil into Groups 1 through 5. Groups 1-3 would be this kind where the origin is crude. In most cases the oil has gone through a hydrocracking process to further refine and create a stable and robust base stock. Group 4 oil is polyalphaolefin (or PAO) or what is commonly referred to as a Synthetic oil. That, however, is a bit controversial as the API considers Group 3 (mineral) to be a synthetic as it has a higher amount of refining and hydrocracking to produce a very high quality base. That said, PAO oils are superior in almost every aspect. PAO's come from a process that uses natural gas to synthesize the liquid. They are also very expensive. So due to this you could conceivably buy a synthetic oil in the US with no PAO. In fact no oil manufacturer will tell you the precise formulation of base stocks, but I prefer to use Mobil 1 as my synthetic of choice for the following reasons:

1. Even though they don't tell you the formula, Mobil 1 does claim to contain PAO. 
2. The formulation of the original hasn't changed in many years. 
3. The overall performance is outstanding regardless of base stock formula. 
4. So much so that they have a high percentage of use in race cars. 
5. I have personally had very good results. 

Additives: There are many kinds of additives.... I will address three primarily. Viscosity Index Improvers (VI), Anti-wear (AW), and Extreme Pressure (EP). VI improvers are a class of polymer that react with temperature to improve the thermal performance of the oil viscosity. That is to say that oil viscosity changes with temperature (shocking I know), and this polymer expands and contracts to make the viscosity change less dramatic. VI improvers are the reason some oils are labeled multi-wight vs single weight. An oil labeled for example as SAE-30W is a single wight and has no VI improvers. An oil labeled SAE 5W-30 would have VI improvers and have the effect of making the oil behave like a 10-weight at low temperatures and a 30-weight at high (operating) temps. There is considerable confusion on this topic, so hope this part helps.

EP additives include mostly Calcium Sulfonates which has the effect of helping the oil withstand very high pressure. An oil labeled as heavy duty, or extreme duty would have more of this.

AW additives are mostly Molybdenum Disulfate which actually has lots of benefits including reducing friction, bonding with metal, and preventing wear.

In addition to the above, motor oil manufacturers often include other additives such as detergents and dispersants to help keep the engine clean by minimizing sludge buildup, corrosion inhibitors, and alkaline additives to neutralize acidic oxidation products of the oil.

One thing I want to emphasis here is that last part. Modern engines use motor oil as part of the emission control systems, and thereby builds up acidic compounds in the oil over time. The counteraction to this would be including a alkaline component in the additives to prevent damage from acidic oxidation. So new oils are always more alkaline than used.

As you can see above, automotive engine oil can be a tremendously complex system, and this is a very simple introduction to the interesting world (I think) of oil.