what weight gear oil for rear end?

[91foxbod50]

Any suggestions? Should I also use l,s additive?

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[87GT Drop Top]

Royal Purple 75W90. It has the additive in it already.

 

[Shakerhood]

The Owners Manual and the Service Manual call for 75W-90, but I believe Ford has issued a TSB that calls for 75W-140 for all 8.8s. That being said I still run 75W-90 with Friction Modifier.