The Shop Manual calls for straight 90 weight mineral oil.
Actually, the Shop Manual has two options for the transmissions: SAE 90 Straight Mineral Oil gear lubricant OR SAE 90 "Multi-Purpose" gear lubricant. The latter being the same that the manual recommends for rear axles.
The question is what "Multi-Purpose" means. There's a paragraph in the manual that attempts to describe it, but doesn't give any real specifications, other than it's designed for Hypoid gear sets (typical ring and pinion).
I would probably go with GL-4 or the straight mineral oil. Although the stuff Phil linked is rated GL-1. Probably OK. It does say it's sulfur free, which is what you want for brass components.
