Since your exhaust valve is the lowest point, all of the oil is draining towards the exhaust valve. The black is probably caused by burnt excess oil buildup. The only way to fix the sticking valve is to remove it and clean the burnt depos**** off of the valve stem. The only way to prevent it is to turn the engine upright. Clarence Lee's column in RCM always has a lot of information about inverted 4 strokes and he recommends that they are not run inverted, however he does say that if you do run them inverted to run them as yours is mounted