With stock shocks, I would suggest running 30-40wt oil. You don't want to go too thick with the stock shocks, it will cause too much pressure, and the plastic shock caps can pop off pretty easily.

A GREAT upgrade for the shocks, is to pick up 2 pair (4 total) of REAR 1/8th scale Buggy shocks. They are a lot more durable, and you can run 4 shocks rather than 8. This saves a little weight, and also makes the suspension a little more predictable.