Nissan says:
* 4-speed gearbox: 1.2 liter
* Japan diff: 0.75 liter
* BW 60 diff: ???
So it seems you will only need 3 liters

Capacities (Datsun 1200 owners manual)



Me? I always had leftover partially used containers on the shelf (I never wrote down how much a 'drain and refill' actually used). But as I recall it never took more than the specified capacity and it may have taken a little less

Also keep in the mind that the steering gearbox uses the same gear oil (GL4) as transmission and differential, but it only needs 0.24 liter. Conversely you could use GL5 in the diff and steering, but the gearbox requires GL4 due to the brass synchros

90W or 80W-90 is good for a bit over 38C degrees temperature. 90W down to 0C, and 80W-90 down to -30C
For very hot climates (40C) use 85W-140