Yep, what he said. I use 75/90 synthetic in mine. I like the way it flows. Easy to pump in through the small holes and should protect well. I know for sure they both shift like butta'. Especially the transfer case.
The original specification called for 50 wt NON-DETERGENT motor oil in the Dana 20 transfer case. Which, according to some fairly knowledgeable types here, is roughly the same effective viscosity as an 80 or 90 wt gear lube. I have always found that hard to believe, but, I'm no viscosity expert either.
What that oil equates to these days, if you want to stick to spec literally, is pretty much limited to a 50wt "racing" oil such as Valvoline or whatever you can find.
I don't know this for a fact, but I believe the factory recommendation was due to additives in detergent oils and then-modern GL-3 (and also GL-4 I think?) gear lubes having properties that ate away at the bronze bushings and shims in the transfer case.
More modern formulations don't have that issue, so I use the 75w/90 synthetic in all four gear boxes.
Original spec for the 3 speed manual was straight 90 weight I believe. But then, they may not even have had the multi-vis versions fully tested at that point. Who knows what they'd specify today.
Your four-speed, if a Ford Top-Loader, probably used the same lube.
Paul