The critical point about using gear oil in this gearbox is that EP90, which you would use for example in the series gearboxes, will be so viscous at low temperatures that it will shear the drive member in the gearbox's oil pump (you didn't know there was one?). You will not know about that until the box is stripped perhaps years later due to unusual wear.
The critical point is not the viscosity at 100C, but the viscosity when moving off at the lowest temperature encountered, which in some parts of Australia can get quite low (and not just in the southern alps or Tassie - I have seen temperatures overnight below -10C in the Simpson Desert, and similar temperatures can happen occasionally virtually anywhere inland, and temperatures below zero are common)

As long as the oil used is relatively low viscosity the differences are mainly in synchromesh performance, and this is the reason why Landrover kept changing the recommendation.

John