Ok, here goes and I hope this makes sense.

I received my Motive 4.56 gears and Detroit Truetrac yesterday. My question is what gear oil am I supposed to use? The manual from the Truetrac says “The TrueTrac differential is designed to operate in those lubricants recommended by the vehicle/axle manufacturer for conventional differentials.”
So that would mean 75-90 for the front and 75-140 for the rear. But the instructions that came with the gears say to use 85-90, which I looked at a bunch of different websites and couldn't find any. And when I looked at Motive’s website, I couldn’t find anything about it, but if you look in their catalog, they only sell 75-140 gear oil. So I am really confused, I don’t know how a certain kind of gear oil gets recommended for a given situation. So I’m not sure if I should use what Dodge recommends. Or since everything is going to be replaced, use what the gear manufacturer recommends.

 

use the 75-140 SYNTHETIC in the rear.

 

Do you have stock or aftermarket gears?

 

Obviously the stock gears need the 75w140 but,--
I'd question that, as a lot of the after market race stuff tells you to stay away from the Fully synthetic. !! I fitted a trutrac with motiv gears in my Mustang and I used the 85-90, with Kendall limited slip axle lube, which they recomended.
If you dont get the definate answer, here, that you are looking for, then call trutrac or motivgear.

 

If your are running brand new gears, run a mineral oil for the first 500 miles and then switch to synthetic. This helps them break in properly.