Synthetic gear oil in old crusty rear end.

[richards72chevy]

I've done a search on here and Google. A lot of people are running synthetic gear oil in these rear ends.But no one is stating if rear end has been rebuilt or if it is the original.I guess the synthetic has the advantage of not needing any additives.If I run synthetic will it jive with the old mineral oil that's left in the rear end? Will the synthetic do anything to the old seals? I'm probably over thinking about it lol.IMG_20200708_171958.jpg