To NicelyHelical Diff
The workshop manual recommends Helical diff oil be changed every 40000 km/24 miles or every 24 months. I says the Viscous oil just needs inspecting regularly, but I'd recommend the same as the Helical.
1.8 litres (overfilling to 2 litres from the top will give a smoother gear change)
The manual also says to inspect and change if necessary for the gearbox. I'd say change at the same time as the diff
Just get a Nissan filter Its fairly cheap and good.
The drain plug key is 13mm