I recently had my rear diff. fluid replaced. The mechanic told me that the friction modifier is in the 75-140W synthetic gear oil. Can that be right? He got it NAPA.

 

Yes, it can be right. In my cars, I've used Royal Purple & Lucas Oil; both come with friction modifier.

 

Well I did the fluid change.
I did not use a gasket, rather I used the silicone as everyone suggested.
I also used Royal Purple 75 W140.
Good thing I did the change, cause I only got about 1 litre of fluid out of the diff. when I opened it.
The truck seems a lot better now when I go from drive to reverse.
Anyway guys thanks for the info.

 

Bottom line is silicone RTV is better and cheaper. It is a much more positive seal than a gasket. It takes a fair amount of force to crush a gaket. The silicone takes less force and will stick to both surfaces provided they are clean and oil free. You do need a high durometer (stiffness) silicone (usually red, orange, or black). A lot of times gaskets will seap over time. Silicone won't let out a drop.