Transmission lube is a touchy subject for a lot of antique/muscle car owners. The old 4 speeds used fluids that were old standards, old Mil Specs, etc that seem to have been consolidated over the years to equal GL4. But GL5 was introduced years ago and is said to be a superior product for hyphoid gears like differentials. There are stories, rumors, myths, supposed lab reports that GL5 will deteriorate brass like blocker rings. But, others say GL5 is the closest thing to meeting and exceeding the original specifications. And although there is said to be some truth to the issue of brass deterioration with GL5 it's said to only happens at temperatures that transmissions don't operate in. And then there are the synthetics and custom lube products.
I experimented with
synthetic in my old Muncie for awhile but the case started sweating the stuff - dry to the touch but always looked like it was wiped down with WD40 or something. I found NAPA still sells GL4 (Sta Lube 85-90) and I bought 4 gallons of it a few years ago and use it now in my two old 4-speeds. Honestly, GL5 is probably of no harm and even superior so I'm comfortable with either one. I don't really think the synthetics are a good idea in the old style transmissions and except maybe under track or heavy use conditions the custom products are not necessary. We are pretty temperate in Arkansas so the 75-90 or 85-90 seems fine. I noticed in my 67 Plymouth service manual the other day that they even suggested ATF in their old A833 4-speeds for cold climates - although how they got that stuff to stay inside one after it had a couple years running time on it I'll never know.
But everyone has an opinion on the subject.