Time to change diff oil on my 1955.
car sat for 25 years
would 75w90 suffice or what are some suggestions

 

Straight 90w conventional oil is preferred, but 80w90 will work. Don't use synthetic.

 

Another option is 85W140. I have been using that in my old stuff like my 70S. Brian Trick uses that weight in pretty much all his rebuilds, he does it for a living. I only use synthetic in the newer stuff that sees -40.

 

The 1955 Oldsmobile shop manual says to use SAE 90 Multipurpose Gear Lubricant.

 

as above, just get some GL5 rated lube.