Howdy. You may remember my car has been parked for 12 years or so. Should I drain the diff or just top it off? Any recommendations for oil to use?

Doesn't cost much to drain it and refill.

90wt Gear Oil ---brand of your choice

why wouldent you change it ? Think about it , was the oil fresh 12yrs ago ? Most likely not . Can water vapor condence in the unit over the years of sitting ? Most likely yes. Everyone knows that can do damage , why not spend a little time change it.