hi i have a gas gas 250 pro i run mine on 75mm to 5lt of petrol i use ipon semi synthetic sill smokes but not alot. i was told only use fully sythetic if your really riding hard. or it may be your carb is runing to rich. have you checked your plug? if its oily it could be rather the carb is runing to rich or the oil is not burning of so you may need to use what i use. if you not riding the bike hard hope this helps