Yes, it does say that doesn't it...

Differentials & Other Land Rover Parts | Differential - Rear - 24 Spline

The diff is the same for the D1 and D2, and yet the D1 manual only requires conventional gear oil. The change probably has more to do with marketing. You can spread out the number of required services buy using synthictic. They also used DEX-COOL antifreeze for the same reason. "I just bought a $35,000 car and I have to pay how many thousand to do the service???"

 

There is only 1 reason any car manufacture uses long life fluids...EPA and CAFE fines.
The more your fleet of cars pollutes the more you have to pay the US Gov to sell your cars here.
One of the ways around that is to extend service intervals, fewer intervals means less waste fluids which supposedly means cleaner air/ground/water, etc.
Another way is to lower the average MPG of your fleet of cars, you can do this by selling small high MPG cars and hybrids.
But if all you sell are gas guzzling SUV's you need to do it in other ways.
Do you really think GM wants to sell the POS Aveo that they loose money on whenever they sell one?
But it gets 40mpg, its $12k new and they sell like hotcakes, so that lowers their overall fleet MPG average which lowers their fines.
The LR4 can go 15k on a single oil change, just like BMW, that lowers the fines.

 

I use Redline 75W140 GL-5 Gear Oil: Red Line Synthetic Oil - Gear Oil for Differentials - 75W140 GL-5 Gear Oil

And yes, from the factory it came with synthetic...mine is an every-other day vehicle, but during deer season I tow a trailer. I also use Redline Waterwetter in the coolant.

IMO, 75W/90 will work just fine, even if you take it off-road moderately, but maybe not extreme; our trucks left the factory begging to be taken off-road.