Rotary valve?
Depends on how much and to what degree your seeing a "black tar" type oil. If your not using much oil, this may be caused by a build up in your exhaust system that is finally shaking loose. It can become concentrated in your exhaust from a slight leak between the rotary valve seals, into the crankshaft area.
Does it smoke when you start it? If so, does the smoke go away after a little riding?
With water mixing in your tuned pipe, if soot has built up in it, it may be globbing up with the water and coming out. If the engine is running well and you are not using any oil, I wouldn't worry about it. You should pull your RAVE valves too, inspect and clean if they are gummed up with carbon.:cheers: