What kind of gear oil for 8 3/4? Why not synthetic gear oil?

The friction modifiers were developed to keep the cone and clutch type diffs from chartering, sticking and making noise. TrueTrac and other types of all gear differentials don't have that problem.


The Chrysler corporate rears with limited slips (9.25 and 8.25) still use a friction modifier with either 75w-140 (for the 9.25) or 75w-90 (for the 8.25) synthetic lube. The pre 2003 2500 and 3500 trucks with limited slip Dana rear ends also use the modifier and the synthetic for refill after service.

The current AAM axles (9.25, 10.5 and 11.5) use the synthetic lube but don't require the friction modifier for their limited slip differentials.

I hope this helps. The friction modifier isn't even a necessary additive for a cone type diff if you don't mind some noise and maybe a little jerking from the rear end at low speed parking lot type maneuvers.