My '87 owner's manual says 2.3L / 2.4 qts. of gear oil for the manual transmission, "GL-4 or higher" but that was before GL-5 was developed and found to be unsuitable for earlier trans designs, so stick to GL-4 oils and avoid GL-5.

I'd recommend Redline MTL gear oil for your Nebraska climate, maybe their MT-85 since I grew up in Lincoln and remember summer days got above 100°F there pretty frequently even back then, or their MT-90 for even hotter climates like down here in Tucson. The heavier weight might also help with transmission noise due to wear, so maybe just go with the MT-90 in your case.