When you clean it are you using motor spray or degreaser? The trick is to get all the oil out of the bearing then apply a "light" oil on the bearing. I know it sounds funny but the light oil helps keep the heavy castor oil out. Traxxas fuel does have a high oil content in my opinion, try Byron's Race 2000 or Trinity Monster Horsepower maybe Odonnels.... bashing fuels have a high oil content. Some other things I have seen work are using steel wool to scuff the starter shaft, make sure you scuff it long-ways on the shaft and not around it. Your tune might also be rich, that will put more oil in the bearing. The exhaust gasket might not be sealed well enough, check that. Even simple things like storing the truck so the oil does not run to the back of the engine.

Hope any of this helps......