Originally Posted by ta04evah

The oil that is supplied in the kit for the diffs is way too heavy for the rear diff. It's fine for the front diff but in the rear it will make the car spin out easily.
Get some 500-700 weight shock oil and use that in the rear diff, it will allow the rear diff to spin freely rather than being hard to turn.
(If you cant get some, use the thinner oil that came with the kit for the shocks.)
If the rear diff is very hard to turn it's almost like having a solid axle in the rear which is what the drift guys do, but not something you want.

When building the diff with very light oil make sure you use plenty of grease on the seals, especially the 2 O ring seals. Also coat the center seal fully with grease and the areas in the diff where the seals sit.
This will help to seal the oil in and lubricate the seals on the out drives, and act as a seal under the heads of the diff screws as you tighten them.

Also make sure you sand the diff housing with the screw holes flat, as that will help to seal the 2 halves together without having to over tighten the screws that hold the diff together.

Lastly, if you're using the servo saver that came with the kit - ditch it. The kit one is rubbish and try to get a better one, the Tamiya high torque ones work rather well.

Cheers
Rob.