I use all my old oils with exception of gear lube and synthetic. I have burned the hydraulic oil from my forklifts at rates up to 20% and the only thing I have noticed is more smoke and little harder start. I'm sure the millage is down but its free fuel so overall its better. I does clog the filter faster even when I prefilter the waste oil. I do run this in both trucks and I dont see any problem.
-robert