I've used it for gear boxes and such that called for that weight gear oil. Not in Buick though, tractors ands equipment and such. I thing I have a partial gallon in the garage stash of lubricants.
I have also used many gallons of that trans fluid in my stuff. I have since learned to buy the trans fluid in a five gallon bucket and put it into the gallon container for easier dispensing.
Apparently Lucas 80W90 has the specs to not require any additives for limited slip. It is API GL-5, but GL-4 does need an additive for limited slip. This is just from what I have read, everyone has an opinion.