hard shifting 833

i have a 68 fs,manual and it states that due to climate you can use either atf or or gear oil.in the old non o.d. tranny,but i just finished putting a 1976 alum,case o.d.833 and i used 80-90 but that will be going out and atf going back in as one of the guys on the forum said that was correct for the car,but i love what it did for my car.