Gear Oil
It is quite difficult in finding the recommended oil for the transmission (M/T) and transfer case. 75W-80, GL-4 has to be used. But I did not find nothing close to 75W-80 other than the 75W-90. So how about using the latter one?
You can use 75W-90. There won't be any issues.
75W-80 Gear oil is used for manual transmissions, especially for service fills. They reliably lubricate heavy-duty manual transmissions in vehicles. To answer to your specific question, 75W-90 is thicker and heavier than 75W-80. If 75W-90 is used in transmissions designed for 75W-80, it may result in reduced fuel efficiency and deterioration of shift feel when cold. On the other hand, if 75W-80 is used in transmissions designed for 75W-90, it may result in gear rattle noise.