Silicone diff fluid that you get at the hobby store is the ONLY thing I would use. I use 50,000 front and rear. Its MUCH thicker then truck diff fluid. It has the consistancy of bee honey. I have never had any leakage with plastic or alloy diff cups. I have opened my front and rear diffs probably 4 or 5 times and as long as the gasket is not torn it has never leaked. But to be on the safe side I would order a few gaskets incase they rip when opening the diff up.