Ugh, this kind of thing just won't go away.

Lubricating oil used in a engine isn't designed to be burnt and we have changed the oil because it is contaminated, why would you want to run it through a injector pump with clearances much smaller than a crankshaft
? Same goes for ATF or rear end lube.

Lube oil has additives that leave ash , carbon and other bad stuff when burnt. Find someone that has a waste oil heater and ask then what kind of junk is left in the combusion chamber and how often they need to clean it out. Do the same for a heater that uses fuel oil. ( Fuel oil / heating oil is diesel fuel dyed red )

Sure you can get away with burning old oil but at what potential cost? If you got the used motor oil clean enough, would you dump it back into your crankcase?