In the various lubrication seminars I've attended, we were always told that "Oil doesn't wear out- - - -it just gets dirty". If oil was prone to deterioration over time, wouldn't the stuff underground decompose before it was brought to the surface? It's possible to "re-refine" used oil and get a pretty good product suitable for certain lubrication duties even after it's been run its supposed normal lifespan in an engine. The sludge and contaminants are left behind, and the base oil goes through a similar fractional distillation process like it did the first time.
Jerry
