Here is a list of the currently available gear oils that match RBT's recommendation. This isn't an exhaustive list. I'm sure there are others but these are the ones I know of. There may also be GL-5 gear oils that are OK (eg. Redline 75W-90NS) but because Lloyd Butfoy says to use GL-4 gear oils, that's what I've listed here. Gear oils that say GL-4+ or GL-4/GL-5 are just GL-5 oils. Lloyd also recommends changing your gear oil annually, regardless of mileage driven, due to condensation forming inside the case. In fact, the less you drive your Pantera the more important this is.
Conventional (mineral based):
PennGrade Classic GL-4, 80W-90
Driven Racing Oils, GL-4, 80W-90
Richmond Gear, GL-4, 80W-90
VP Racing Classic, GL-4, 80W-90
Synthetic:
Redline MT-90, GL-4, 75W-90
Pennzoil Platinum, GL-4, 75W-90
Amsoil, GL-4, 75W-90, Manual Transmission and Transaxle Gear Lube
Motorcraft (Ford) XT-M5-QS, GL-4, 75W-90, Full Synthetic Manual Transmission Fluid
Note: a ZF Transaxle is more likely to develop leaks when using synthetic gear oil. If you're using synthetic oil and you do develop a leak, it doesn't necessarily mean that you have a bad seal or gasket. Synthetic oil is just better at finding a way out than mineral based gear lube! Lloyd Butfoy tells me that even brand new, freshly sealed, ZFs sometimes leak when filled with synthetic oil.