Hello,
It looks like I might need to rebuild the differential on my 85 caprice. Its only got 185k miles on it./ Before I do I want to change the differential fluid. Easy enough to do but I am not sure of what type of gasket to get to replace the old one on the differential cover. Its a 10 bolt, I know this much. What else will I need to know?

Look for a small metal tag under one of the bolts. Take this number with you to the parts store. You can always clean the cover and take it with you.
Counter help isn't as experienced as they used to be.

Hello,
It looks like I might need to rebuild the differential on my 85 caprice. Its only got 185k miles on it./ Before I do I want to change the differential fluid. Easy enough to do but I am not sure of what type of gasket to get to replace the old one on the differential cover. Its a 10 bolt, I know this much. What else will I need to know?

More than likely your car has a 7.5" on it. The 7.5" cover looks like a 'round square', while the 8.5" is a true oval shape.

As for myself, I don't use gaskets anymore on diff covers - I use "Permatex The Right Stuff". It seals better.

I have heard that on the 8.5 its best to use a gasket due to some holes designed to help move lube to the wheel bearings.Sounds like the fel-pro gasket is the prefered one.Not meaning to disagree with silicon212 I found the info on a 9c1 site.