What engine oil are you using? If you're going through a lot at the moment you might want to look into Rotella T4. It's formulated for diesel engines but don't let that put you off. The weight (15W 40) is good for our engines too and is formulated for wet clutches. I got turned on to it by a lot of bike guys as a much cheaper alternative to the more expensive moto-specific oil. Used it ever since on all my bikes and never had a problem.