Posted by Traveler on January 24, 01 at 13:36:55:
In Reply to: Friction modifier for locking differential posted by John G. from NC on January 24, 01 at 11:47:27:
You are fine. The friction modifier additive is only needed for "dino" oils (non-synthetic). Also the need for it is less in locking differentials than limited slip types.
The new GM non synthetic diff oil has the additive already in there so there is no need to add it separately. The additive is sold for people who are either not using the GM brand oil or for cases of unusual chatter, probably caused by worn parts but usually fixed by the band-aid of adding more posi lube.
For future services I'd suggest using Mobil 1 or AMSOIL gear oil and following a 25K mile +- service interval (shorter if towing).
You should be servicing the front diff at the same time as the rear as well as the transfer case. The T-case takes special blue fluid from GM only, if you have auto-trac. Otherwise the T-case gets (synthetic) ATF.