Undo the fill hole on side of gearbox, undo the bottom drain plug, and let the oil run into a bucket.
Tighten up the drain hole first, and then get regular gear oil (se GL-4 80W-90 or 85W-140 multiweight) and using a container with a pump and hose, fill the gearbox with oil until the oil just starts to come out the fill hole on the side of the gearbox. put plug back on side and tighten up.

see:
http://datsun1200.com/modules/mediawi ... ssion_Overview#Lubricants

Hey guys i got a standard 4 speed box in my datsun 1200 ute, ive owned it for rough 7 months and dont know how to change it, cam someone give me advice on how to & whats the best oil for it and how mucu litres?