I wouldn't put any more gear oil into the box than that which is already in there. There is a top plug, which is the vent. If you look at it carefully, you will see there is a very small button that you can lift. There is also a side plug on the gear box. You remove this plug and fill from the top till gear lube comes out the bottom of this plug hole. Then it is filled to the proper level. Only use 90 weight gear lube in the box. Synthetic would be better than Dino, but either way, both will work. DO NOT use engine oils unless it is an emergency and then only the heaviest of weights and change it ASAP. Don't forget to grease the fittings on the augers..... one on each side.