I know the diff is often neglected when it comes to fluid changes. How many miles should a car be driven before the fluid needs changed? Also I am replacing the pinion seal in my diff and am going to fill it with Moroso climbing gear lube is this alright to use??

 

i'd change it once a year more than depending on milage. i'd use a good synthetic gear oil, like amsoil or mobil and gm posi additive, if you have posi.

 

The only synthetic I can ever find is 75-90, instead of 80-90. Is that ok to use?

 

I will be putting 75W/80W Red Line in my rear end sometime over the Christmas holiday.

Note that GM issued a TSB in Oct 1998 saying the the Brog Warner 9-Bolt should NOT get the posi additive.