Reinstall the cover, and fill with fluid. We chose Royal Purple synthetic 75-80 Max-Gear lube. Max-Gear is recommended for automotive differentials and manual transmissions. Max-Gear is a tough, high-performance, GL-5 automotive gear oil designed to provide maximum protection to heavily loaded gears. Max-Gear also has hypoid friction modifiers for clutch or cone-style differentials already blended in the mix. This means you don't need to buy that also.