Diff fluid
Meant to add that you might want to try 10,000 to 15,000 in the front with 5,000 to 7,000 in the rear.
I run roughly have of the front in the rear.
You will have to experiment and see what you like. If you race and have lots of extra parts, its easy to load up a couple of extra diffs with different weights. Then at the track you can just pop them in to change the weight for testing.
Dbow