I've ran plain ol motor oil, ATF, type f, 80w trans lube, 15w40 diesel oil, etc. Your clutch has to be pretty jacked up for one of those fluids to screw it up. You're better off with a specific trans/clutch fluid for the piece of mind. Some of the 4 stroke guys swear by the diesel oils. If they can run it in the entire engine we can run it in our trans. And most of that is pretty cheap.