Elmo, GL4-rated fluids are recommended for transmissions. They promote smooth shifting and are synchronizer-friendly, especially important for gearboxes that may have "yellow metal," brass/bronze components. The mineral oil recommended by Mike B. is rated GL1, and OK for older transmissions that don't require a higher level of protection. GL4-rated fluids are better products in that regard, but generally cost a bit more. My go-to is Red Line Oil MT-90 which is GL-4 rated, FWIW, since it's what I keep in my shop to service several newer vehicles.
GL5-rated fluids are recommended for wear protection of the hypoid gears used in rear differentials. Once again, FWIW, I use Red Line Oil 75W90 or 75W140, depending on what's called for, and because it's what I have in my shop for the same newer vehicles (BMW, Nissan).
There seems to be a bit of debate in the forum, but the sulfur content in GL5 fluids may promote corrosion of brass/bronze components, and they are generally not recommended for transmissions (which do not use hypoid gears) and so the increased level of protection isn't needed.
Hope that helps.
Doug
