HPI customer support is excellent.

The shock oil is going to depend on your driving style and driving surface. You might have to try a few different weights till you find one that suits you.
The benefit of the diff oil over grease is you can adjust the amount of action the diff has by using different weights. Heavier diff oil will limit the diffs giving more of a posi traction where the lighter will allow full diff action. Again you can adjust this to match your driving style and the track you run on.