Ken or Jerry should have more info on this but it’s my understanding that the biggest
enemy to oil longevity next to contamination, is heat.
Consider that there is a light film of oil present on the cylinder walls and on the piston at
all times. So every time the cylinder fires it burns off a tiny bit of oil the ash is eventually
washed back into the main pool of oil.
In petroleum oils with lower flash points the smaller molecules in the oil vaporize and
burn off more rapidly than the larger ones. The larger oil molecules do not flow as easily
and tend to coat the surface restricting the oil flow which would normally carry away
some of the heat thus adding to the heat problem. I also recall hearing that the pour point
depressant additives in multi-grade oils that keep the waxes from solidifying are used up
quickly, which also contribute to oil thickening up. So does petroleum oil break down
when subject to high heat in an infernal compression engine, sure does.
The synthetics stand up to high heat much better because they are compounded to have
that same size molecule so even as some of the oil still burns away it remains consistent
in its flow characteristics. Contamination heads up the list of reasons to change synthetic
rather than compound breakdown and its not unusual to see recommended oil changes in
the neighborhood of 10,000 miles using synthetic oils and a good filter.
And again, the transmission and rear axle is not subject to high heat like we see in the
engine so there is very little break down in the oil over time.
Denny Graham
Sandwich, IL
Last edited by Denny Graham; 12/02/2011 8:58 PM.
