What Diff oil for a Sierra.

Ime planning on checking out the rear diff this week (in Situ) and am wondering what oil to use for topping up,do i need to drain the old oil as its covered around 320,000 miles.
Diff is in a 91 Sierra 1800.
Thx

trevs said:

it would be a good idea to drain the old oil first, you will have to take the back plate off as they dont have a bung for draining imho. I think you need ep80 or ep90

Checked it out this afternoon aand as you say there is no drain plug ,also used the 90 grade.

The hollow filler plug was well gunged up with grease and there were a lot of metal shavings on it,so i cleaned it up and flushed new oil into the diff.

Once i get a gasket ile do the job properly but at least its topped up.

Cheers.