between the different cars I own (W107, W124, 2 BMW's) the requirement for differential fluid goes from 90 sae , 85-90 sae, 90-145 sae. of course the only fluid available at Pep Boys is 80-90.
Can I use one grade that will work for all differentials - if it exceeds the grade needed will it cause harm?
On one of these forums I read a mention to use Swepco fluid as the only fluid that will truly eliminate differential noise. Well, normal 80-90 stuff costs around $7 a bottle. other grades available on line are around $15/bottle. And then there's Swepco - at $45 / bottle.

A little guidance please.

thanks