A neighbor ran used oil in a GM 350 diesel for several years with no trouble. He did set up a barrel with filters dripping into a second barrel, and put a huge filter on the truck. The engine went away after a few years, but it was a POS to begin with. He ran the used oil straight, by the way, no dilution. He claimed 20 more horses at least, by the seat of his pants.
Some Cummins engines in semi's use an on-the-go oil change system where a photocell reads the colour of the oil and at a certain point bleeds off the old oil into the fuel tank and replaces it from an on-board tank.
Caterpillar says to use crude oil as fuel if regular fuel is not available. The warranty may be voided, though, due to unpredictable power increases damaging the engine.
Like the others say, it's your engine, and your nickel. I'm going to find an old diesel pickup and try it, I have hundreds of gallons available.