4 Speed gear oil

slant6/4speed said

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.

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Not so. Been running 4 speeds for almost 40 years, built them too. No problems you have mentioned, unless the trans was worn badly. A trans with Auto trans fluid will be worn badly in about 100k, 90 wt gear oil will run forever with almost no wear 4 speed or OD. Never had slow shifting!!! It's your trans do what you will. At -20 degrees it was a bit slow till it warmed up, now that I think of it...