85,

Because the detergents in the oil are consumed and some of the oil polymerizes from the heat and in DIesel engines, the soot level increases beyond specifications.

Then is time for new oil in the crankcase and the old oil can be used to power the Diesel engine. Its only going thru the engine one more time after going thru the engine thousands of lubrication cycles. And remember I filter it before I burn it.

P E H