Front Differential Oil?

'96 LGC, 5.2 V8, Quadtrac
The manual says to use 80/90 gear oil in the front diff. Can I replace with 75/140 synthetic? If so, does it need the additive? On the other hand, the rear calls out 75/140 syn. Does it need an additive?

Thanks

75/140 all around is fine. You only need a friction modifier if your diff has a locker. Unless someone installed one in the front no worries there, the rear may or may not have one factory.

'96 LGC, 5.2 V8, Quadtrac
The manual says to use 80/90 gear oil in the front diff. Can I replace with 75/140 synthetic? If so, does it need the additive? On the other hand, the rear calls out 75/140 syn. Does it need an additive?

Thanks