Dabesiv

iTrader: (1)

I hate to say it but in the 8 years I have owned this car I have not once changed the dif fluid, partially because I had absolutely zero mechanical knowledge when I first got it. Now I am confident in being able to change it, but I am terrified of what I might find.

I asked the original owner and they never had it done either, so I could possibly have some 34 year old oil in that thing, and who knows what else.

Any precautions I should take into account? or a recommended brand of oil?

big gear head

The very first thing I would do is to be sure that the fill plug will come out. Many times these get stuck and require a lot of work to remove. Several times I've had to weld a bolt to the plug and use a wrench on the bolt.

If you get the plug out then you can remove the cover and drain the old oil. Clean the cover and the sealing surface of the housing and install the new gasket, RTV or whatever you are going to use to seal it. Put the cover back on and fill it with oil. I would recommend a good brand of 80W90 gear oil, like Valvoline. If this was a new rear end or something special I wold recommend 75W90 Mobile 1. If you have the G80 limited slip differential then don't forget to use the limited slip oil additive from GM. Put it in first.

85FBTranam

iTrader: (3)

Make sure your fill bolt or rubber plug will come out before you drain or you won't be able to fill it back up later. Rep are for it to smell pretty bad.

Go for a synthetic and add a friction modifier if it has a limited slip. I used oral purple which had it installed already.

Lastly you may need a new gasket for the diff cover.

Matt trans am

Does royal purple have the additive already in it?

big gear head

Most of the time I have had to use the additive even when using an oil that says it already has the additive. The only way to tell is to drive it and put the additive in later if it doesn't work. I used Amsoil in most of my customer's rear ends for about 15 years and it worked well without putting in the additive, but that is the only oil that I found that worked without using the additive.

1989gta

Majority of reputable gear oils are compatible with limited slip rear ends. I just did mine used Mobile 1 it says it right on the bottle.