Oil Change Intervals

Oil suffers several different degradations in ordinary use, some of which I will treat here.

Diurnal cycle
When you don’t drive your car for a couple days, the massive engine stays cool relative to daily high temperatures. As morning air warmed by the sun comes in contact with the cold metal, moisture condenses out of the air onto the engine, just as it would onto a glass holding an iced drink. Some of that moisture enters the engine’s interior through the venting system. There it causes rust, brings in other corrosive agents, and is a poor lubricant.

Combustion cycle
The combustion process causes oxygen and nitrogen to combine. The resulting compounds are acidic. Piston rings are not perfect, so some of this acidic solution is forced into the sump and incorporated into the engine oil. Other problematic combustion products include water and carbon particles. Carbon particles clump together in the combustion chamber and the intake manifold, where they can dissolve into the oil, forming an abrasive slurry kind of like toothpaste, and circulate. This slurry is pretty terrible stuff, and has been observed here to ruin oil seals prematurely by abrading the point of contact between the seal and the surface it sits on. (Those seals are also damaged by the engine overheating that can result from low oil or coolant levels.) Finally, carbon also lodges in the lands of the aforementioned piston rings, gluing the rings to the pistons and making them ineffective. Long drives (over 10 miles) not involving long idle periods (traffic) allow the engine the warm up and push out some of this bad product, which is why Toyota’s owner’s handbooks of the 90’s and 00’s typically predicate the oil change recommendation on the length of the average trip.

Duty cycle
Most engines hold about four quarts in their sump but only require about one quart to fully supply the whole engine. This means that any one molecule has a about a one-in-four duty cycle: it spends three time units resting for every one working. (Not far off of my own duty-cycle.) When the oil level decreases, the duty cycle increases, and not surprisingly, things start to fall apart. As the work load goes up, the oil temperature increase, and the rate of ‘cooking’ increases- and the consumption then increases further, so that these effects compound one another. The result is that oil consumption over distance, absent additions, is alway steeply curved. (Engineers that have reviewed this article say that they are unaware of this: in other words, skeptical.)

Longevity
Show me a Toyota or Honda with more than 200,000 miles on the odometer that doesn’t drink oil, and I will show you an engine that has known only clean oil. It is that simple. Here is a simple tool for calculating the proper oil change interval for your car.