you misread my post.

once oil in the crankcase comes in contact with combustion byproducts (gasoline, blowby, etc) it starts to break down (thats what sludge and other associated things are) why do you think oil gets dirty? Byproducts, material wear, etc.

if left sitting in oil that never gets run up to operating temperature, the oil beings to break down.

I've seen plenty of built motors croak because someone left oil sitting in a motor over a long period of time. A buddy of mine had a built motor in his drag car, left it sitting with the same oil in it from the last runs he made, went to crank it up and as soon as the motor turned over it spun bearings, no oil pressure because the oil had broken down.