Most conventional drive axles use gear oil viscosity of about 80W or 90 grade. Consult your owner's manual for more detail. Limited-slip or Positraction® rear axles usually .require a special lubricant or in some cases, it is available as an additive, which is available from the dealer or parts store. If you do have a limited-slip differential, make sure you use only the correct lubricant, as the use of the incorrect lubricant can destroy the differential.