Depends if it's got an LSD. If not and it's not been changed since new then I'd get it nice and warm and drain it, re-filling with a good quality synthetic gear oil such as Motul 300 75-90. If, however, it has an LSD it needs changing more frequently, at least
every 20k miles because of the wear particles which can adulterate the oil. In this case it needs a different oil compatible with mechanical LSD's such as Motul 75-140 Competition. If you dont know whether the car is fitted with an LSD just jack up the rear axle and turn one wheel by hand. If the other wheel turns in the same direction you have an LSD. If it revolves in the opposite direction you dont. Hope this helps.
