I've Never done this, and the teck wiki doesn't explain how to do it.
Thanks

There's a square nut on the top of the steering box .... remove it and fill with gear oil

Great thanks, hopefully it won't steer like ww1 tank after that!
3/4 fill from what i've read?

fill to the top? It uses GL-4 80W-90 or 85W-140 multiweight, same as the Transmission oil

But if you want to drain out the old oil ... remove the box/column, turn upside down. It's not a normal service item which is why there's not an easier way

The drain plug will be the square bolt. Don't remove the one with the nut and slotted bolt through it , as this is used for setting correct backlash on the sector shaft...if you adjust it wrong you could find the steering binding up when you corner.

The Gregory's manual I have recommends SAE 90 oil.

Yes, Datsun also recommended 90 weight. Or 80W-90 or 85W-140 multiweight. It's in the Owner's Manual

The square plug is on top (fill plug), how can it drain the gearbox? Or is there another?

Good pictures. I'm assuming there is no other square bolt and you may need to suck the old oil out?
In terms of that adjustment nut and bolt i take it if it is screwed in this reduces play. If screwed up too much it may bind as you said.

I just read on an old post that the box takes 250mL. If this is the case, a big 100mL syringe with a bit of vinyl tube jammed on the end could easily suck it all out.

I just checked the level and it was low. I used a syringe with a pipe as suggested and it worked brilliantly to extract old oil and put in new oil. Filled up to top.
Another bonus is the steering is much lighter now!