I believe it is a good idea for a serious racer to carry a 2nd set of diffs with a different setup, that will make it easier and faster to test different setups at the track. For the front diff, we tend to run oils between 5000 and 10 000. The standard setup is 5000. Thinner oil gives more off power steering, so more steering into the corner, and a more responsive nature, but less on power steering, and steering and pull out of the corner. Thicker oil does the opposite, less off power steering, more stable with less steering into corners, but more steering and pull out of corners.