I use waste motor oil (WMO) in a Listeroid 6/1 stationary diesel engine (1920's technology, 600 rpm, 5 inch piston, 800 lbs, and only 6 hp). Start and stop on red diesel, then switch to WMO when hot. Carbon deposits will form on the injector which needs to be cleaned every so often, but that's easy to do. I let the WMO sit in 55 gal drums for >6mos to settle out as many particulates as possible, then it gets thinned with about 20% diesel. After that I heat it in a 40 gal old water heater tank, then centrifuge it at 8000 rpm. Great free fuel! Time will tell how long the engine lasts...
Marcus
how are you centrifuging the oil? just small batches?
I have been thinking about going off-grid with a listeroid 6/1 to build away from lines. The other option I've been playing around with would be a comparable Arrow engine burning propane (or maybe even wood gas). I figure with either option I'd capture waste heat from the engine to help heat the house.