Tornado Differential grease
The function of a differential is allowing the outside wheel to travel further than the inside wheel in corners. The differentials include a limited-slip function that prevents the wheel that might be slipping from spinning out of control.
Using high viscosity silicon oil (thicker fluid) you set the slip function tight to prevent the wheel to slip out of control.
For example on an off-road track. On the other hand, for tracks with a high degree of traction, you may use a lower viscosity (or no oil) so the car slides more naturally through a corner.