gear oil actually comes in a caulk like tube. you remove the filler bolt, and basically squeeze it in. it's practically impossible to overfill, unless you are able to hurry and put the filler bolt on . there are fancy devices that measure it for you, but it's not required; just fill it and let it run till it doesn't come out. to check it at first though, just remove the filler bolt and put a pinky in the hole and see if you have any oil on your pinky. hope this helps.