Diff oil is the silicone fluid inside a differential. Its thickness controls how freely the wheels turn at different speeds, which affects traction and steering.

Thicker oil tightens the diff for more forward traction, thinner oil frees the car up to turn more easily. Tune front, centre and rear separately to balance handling.

Change it when the car feels different or after a rebuild. Racers refresh it regularly, while casual drivers can go longer between changes.

It depends on the car and track, so check your setup sheet. As a rule, a thicker centre diff adds stability and a thinner front adds steering.