Factory settings for the hyper 8 is 3,000 weight oil front and center with tacky grease in the rear (1k oil is a good replacement for it). If you haven't even driven the buggy yet I highly recommend you do. If you start changing the settings right out of the box you don't have a good starting point to work from.

I'm not sure what your talking about by having the diff leaking. I don't have a hyper 8 myself so I don't know what diffs are in it. If your talking about the center spider diff then keep the spur gear on and unscrew the back side of the diff but leave the screws in on the gear side. Put it flat on a table to hold the screws in then fill up the diff 3/4 of the way. The front and rear diffs are only 2 piece instead of 3 piece like the spider diff. For those just take off the ring gear and after cleaning out the old oil/grease out just put in the new oil and close it back up.

I always fill my diffs 3/4 of the way just so there is minimal air in there. It makes the perfomance a little more consistant (bairly enough to notice)... its more of the way I like to do it over any type of performance gain . The diffs are designed to be full of oil though and I never had a diff blow because of it.