Fox is right, I've been putting in the proper amount since day one, never had oil in the air box.
This is a little off topic, but would it be ok to not connect it to the airbox, and just use a little crankcase air filter like this? Maybe you'd have to use a bit of tubing in between.
Mine has one of those hidden discreetly behind the air box
Does it need a one-way valve?r80rt, ever have any oil seeping out of yours? I don't have this problem now, but I guess some others do (likely from over filling the oil).