Your '77 has the Triumph 1500 engine and all-syncro gearbox. These cars are fine with normal 90W gear-oil. You can also use the new-style multi-weight 80W-140 gear oil. I use the synthetic 80w-140 gear oil in my '78 race car.
Older Sprites and Midgets (with the 1275, 1098 or 948 engine) have a differnt trans. They should use normal motor oil in the trans. 10W-30 works fine in these older cars.