well, being picky here, the oil doesn't evaporate and the molecules don't become smaller. However, when you spray oil from a nozzle for a purpose such as the oil spray cooling of the pistons, some of the oil becomes aerosolized. That just means that some of the oil droplets are so small that they won't settle out by gravity. The question then is what happens to that portion of the oil, and is enough oil becoming aerosolized to account for the oil loss. And why does it happen in some engines but not others.
If the oil becomes aerosolized, then it is easier for it to scape through small holes in the engine if some parts havenít been sealed completely.
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