I just purchased a '87 Firebird Formula, and noticed the lower half of my 9-bolt cover is covered in a layer of oil. I was told the rear gasket probably needs to be replaced.

Of course I will also be changing the fluid then as well. I was told at a local Pep Boys that since the differential is 15 years old they don't recommend using a synthetic fluid since the gasket near the driveshaft may be a bit worn, and synthetic would perhaps cause a leak to start. Any truth to this? I was going to use Valvoline 80w90 GL5 oil with the GM additive. Is this the safer bet for now? I really don't have the money now to get the front seal replaced although sometime I know it should be done, and then perhaps switch to synthetic. Thank you for the help.

 

synthetic will or can cause old seals to leak. i put synthetic in my old truck last year when i got it and nothing started to leak. there is a chance your leak is from oil coming out the breather and not the gasket. wouldn't hurt to change oil and additive and to take a look around.

 

ede, if it is the breather what would I have to do to fix that? Thanks for the info.