Yeah I was going to say that but you beat me to the punch. There is no way to "check" final drive oil. The manual will say "look in the fill hole blah blah blah", but this is false!!!! If you are leaking out the breather, it is over-full. You will need a quart of 80w90 gear oil and a 60cc syringe commonly found at animal feed stores etc. or medical supply stores. Warm the bike up, remove the bottom 12mm drain bolt for the final drive and let it all drain out, spin your wheel slowly to get every last drop out. Fill up with 180cc of gear lube. Done.
