1950s Correct gear oil 57 cadillac rear diff axle carrier

[Carcadillac]

Hi, I see the manual suggests 90W gear oil, I can get GL-1 or GL-4 oils in 90W, any suggestion on which GL" is appropriate.  It does have some soft metal parts in the unit.  1957 fleetwood 3.36 ratio.

[Chopper1942]

I would recommend ACDelco Axle Lubricant 88900401. You can get it at any GM dealer and online. It is a full synthetic gear oil and can be used in posi diffs.