Honda sent out a bulletin on this to the dealers. Most likely cause of this is starting the engine and reving it right away or starting it and taking off w/out letting it idle first. The crankcase breather is on the right side of the motor. If not allowed to idle, oil will get pushed back into the airbox. I'm not saying this is definetly your situation, but this is the #1 reason for oil in the airbox on a 350 Rancher.