Could be several different things. First thing is wash it real good. Power wash, car wash, spray bottle with Dawn dish soap and garden hose. Then get some flour, talcum powder, baby powder. Ride it a bit then dust it. No dark wet powder ride some more. Keep doing this until you find the leak. Could be head gasket, leaking pipe at flange, possibly leak from clutch cover or oil pump.
Could even be hole in expansion chamber.