NAPA still sells GL1 SAE 90 gear oil in gallon jugs. Tractor supply stores should also carry SAE 90 gear oil. The old Borg Warner 3 speed overdrive transmissions specify the use of a non EP oil along with many farm implements. I use a good SAE 40 engine oil in the old transmissions when I don't have the SAE 90 on the shelf.
Its the correct fill for a Yugo/Fiat 128/Fiat x19 4 speed. The synchros did not care for the super tech 20w50. When driving a Yugo with 55 HP, missing a shift because the synchro didn't let me in could get me killed.
I'd look for an industrial user and offer a contribution to their coffee fund for a quart or two of their ISO 220 viscosity gear oil. This is often used in gear boxes with bronze worm gears and other industrial usages.