Understanding Engine Oil

Did you ever think about engine oil? Most people don’t until it’s time for their 3750, 5000, 7500 or 15000 mile oil change. But the fact is, engine oil is an amazing product. Did you know that the average engine oil has over 200 chemical compounds in it? Do you know what the difference between synthetic and non-synthetic oil is? Or what those numbers like “5w-20” mean on the oil bottle?

I took my car, an Infiniti model, to my local Infiniti Express Service dealer for a quick oil change and service. As an engineer I appreciate engine oil for the amazingly evolved product it is, and I appreciate my dealership because I know that the people working on it are Infiniti certified technicians who only use genuine Infiniti parts and when doing their fast oil change use only Infiniti specified oil. Here’s why this is so important.

Engine oil has two jobs, lubricate moving parts and cool the hot parts of the engine. The lubrication part is done by the oil creating a film, a barrier, between moving parts such as the piston and the cylinder wall which prevents them from actually touching and coming in contact. If the oil film breaks down, then metal on metal contact occurs and if this continues the engine wears out prematurely. The cooling part is accomplished by oil splashing on hot parts of the engine like the underside of the pistons, absorbing heat from there and carrying it to the oil pan where it can be convected out to cooler air flowing under the car.

Your engine oil must flow almost instantly when you cold start your engine. If the oil is too thick and flows slowly then metal on metal contact is occurring while the engine internals are waiting for oil flow to arrive. Once the engine is fully up to operating temperature, engine oil is typically around 220 F degrees. When hot, if the oil is too thin, then again metal on metal contact occurs and engine wear accelerates.

The solution was the invention of multi-viscosity oils. In the old days, oil had a single weight, say 40w. The “weight” of the oil refers to it’s relative resistance to flowing through a narrow orifice. You can thick of ‘weight’ as being the thickness or thinness of the oil. A single weight oil, like 40w, is a heavy, thick oil that at 100C degrees (212F) is certified by the American Petroleum Institute to meet the standards for that weight. The problem is, in order not to thin out at 100C temperature, 40w is so thick when cold that it flows, well, about as good as molasses. If you try to use a single weight oil that’s thin enough to flow well when cold, it will become too thin to keep its integrity at high temperature allowing the oil film to break down.

Multi-weight oils like a 5w-20 have special additives that allow the oil to be thin when cold and act like a thicker oil when hot. The “5w,” the first number, means that at 0C degrees (32F) the oil flows like a thin 5w oil. This is good, it means that when you start your car and all of the oil is in the pan instead of the engine, the oil will flow quickly and begin protecting engine parts. The second number, “20,” means that at 100C degrees (212F) the oil has resisted thinning out so that it has the ‘thickness’ that a single weight 20w oil would have when hot.