Gearbox oil will be OK. Just use another brand for the diffs if you can't find the Castrol. Nissan reccommend Hypoid gear oil GL5 85W-90 for the front diff (approx 1L) and LSD (if you have a mechanical LSD) GL-5 80W 90 for the rear diff (about 1.5L) You could use something heavier for Australian conditions or Motul if you want to splash out!