75w140 vs 75w90

[BrookhavenTuner]

Have had differnt people tell me that either is what to use in the car with a friction modifier, but no one has said what teh differnce is. Is it just that the 90 acts lighter then the 140?

I know a few 01 cobra guy run 90 w/o a problem, but I dont know how that replates to us.

 

[scotmach]

I would use what the owners manuel states and thats 75w-140. Thats what I used in mine and I have a Ford Trac-lok 31 spline and 31 spline Superior axles.

 

[WHiTE03MACh]

I worked for Ford and my buddy is a rear end genious. I believe its something to do with the clutches? But full synthetic 75w-140 is all he and I use in mustang rear ends.