4 Speed gear oil

80/90 is too thick. Synchros will be gummy. Shifting will be slower and hard. May not stay in gear while coasting. Look at the input shaft and the gears on the main shaft/counter shaft. See the little hole every 4th tooth or so? That is a oil pump that pumps oil to the main shaft and the counter shaft needle bearings. Factory manual says ATF type A/ Dextron.