I have a 1989 firebird formula 350, what gear ratio is in the rear end and what Gear oil should I use when I change the fluid?

Thanks

 

Look for a tag (s) on the rear...or look for your RPO codes-should be under the glove box lid (is on my Camaro) for the ratio(to decipher).

Gear oil is kinda dependent upon preference...85-120, 90 wt, etc...a lot depending on driving habits..I/e you take long trips, or is it a grocery getter?

If it has posi (will be evident by have two tags on the rear end cover if unmolested) you may need to add a GM friction mollifier if your oil you buy doesn't have it...most don't.