Madison Sully
Member
OK, I'm awake again. Didn't mean to ignore you last night but I wanted to put a bit of thought into this. So here goes....
When you first start your engine, you get more blowby around the piston rings, and the oil and rest of the internal bits are cool, so water vapors from the blowby (and fuel too) condense in the engine and contaminate the oil. Oil is, of course, blended to mitigate damage from this, but these ingredients are not without limit. Once the engine is hot there is not this tendency to condense water vapor out of the blowby; it all goes out the EGR and back through the intake to be burned in the combustion chamber.
The water in the oil reacts to form acids, which is why oils are blended with ingredients that make it basic in nature; this is why there is a TBN, or Total Base Number. To neutralize these acids that form. But again, once the oil and engine are at operating temperature, the water and fuel in the blowby don't condense into the engine case.
Then there's the estimate that ~90% of an engine's wear happens at cold startup. Few reasons for this, but the increase in water content in the oil is one of them. Another is due to wear caused by lack of oil film until some few revolutions have happened, which will generate particles of wear material due to mechanical wear. These particles will also cause more wear until they are caught in the oil filter, another reason for longer oil life with longer "miles per start".
As to keeping oil in an engine for a few years of low usage, some folks would say that's bad because an engine "breaths" and will collect water from the air. Sort of true, in the sense that an engine is like a slightly open bottle of oil as opposed to a tightly sealed bottle of oil. But it's not like there is a little fan that blows air around and a means of collecting water vapors into the oil. In this case, the engine is at ambient temperature, so not much tendency to condense water, especially on the internals. I'm speaking of a vehicle stored in a garage, by the way. Out of the sun, fairly stable temperature, that sort of thing. I suppose being in the sun would be OK.... Anyway, If the car is in a high humidity place that also has wide temperature swings this may be an issue. But for someone like me who keeps his motorcycles in an attached (but not climate controlled) garage, I'll leave oil in for 2-3 years and not worry. Brake fluid is not a good comparison here because it is hygroscopic; it will actually collect water from the vapors in the air. Motor oil is not like this (or at the very least, is orders of magnitude less likely to collect a given bit of water vapor). Slightly opened bottle of brake fluid you throw away tomorrow for this reason. Opened bottle of motor oil? No worries.
