acidic (used) oil in the engine over a prolonged storage period is the main/rod bearings and other bushing type bearings. The acid lays on these bearing surfaces and creates little "worm" trails (easily seen when tearing down an engine with poor maint.) These "worm trails" are a major instigator of eventual bearing failure and the chief reason I never lay up any engine with old oil in it. When purchasing expensive, used, heavy equipment our company will often pull the pan and a main bearing cap to check for just this anomaly - much more accurate gauge than the previous owners maint logs.