I'd just run the kit 700cst. However, there are other setup things I'd worry about before the diff oil. 700 is a good start as the diff action will be very light and smooth and will help keep the car planted under power. Thicker oils, 2000cst or so, may start giving you a little onpower oversteer. Think of a locker/spool in the rear axle of a real car, when it's locked it's easy to do cookies, when it's not it will tend to push. Obviously, there's a grey area for everything though.