Not to
but...
Both Jay Leno (known for his love of old vehicles) and Redline recommend 75W90NS oil in these older transmissions. Its worth noting that GL4 and GL5 are performance ratings, not formulation ratings. What's in one may not be in another.
Most major manufacturers stopped using formulas with active sulfur EP additives to avoid pitting yellow metals (they do include inactive sulpher EP additives that do not affect brass etc). In general the API GL-5 Multigear EP SAE 80w-90 oil by Exxon-Mobil, Chevron, BP-Amoco, Castrol, Valvoline and Pennzoil would be safe for yellow metals. Also, well-known companies such as Redline, who make specialized racing lubricants, would have recommendations that don't eat yellow metals.
Stay away from other brands that are no-name or house brand. They typically contain active Sulfur, Chlorine and other undesirable additives.
Thats what I found in my research. I'm putting the Redline in Cecilia when I get her back up and running.