I have recently gone through this process of sourcing oils for the front and rear diffs, centre diff and transmission for my Audi SQ5. I ended up buying the (very expensive) genuine oils due to reading some people having issues after using non-genuine stuff. Although a very small risk, I decided it wasnt worth the risk for the sake of a few hundred dollars.

The oil for your diff should be G 052 145 S2 which has superseeded the G 052 145 A2 G052145S2 - Axle Oil (1 litre) - Differential & Gear Oil - Audi & Volkswagen – VAGPARTS Australia

There are several brands that are are apparently able to be used, but only the Liqui Moly is easily available in Australia: LIQUI MOLY - SAE 75W-90 (1L) - Manual Transmission Fluid - VW 501 50 (G50) – VAGPARTS Australia

I bought mine from ECS Tuning in USA along with a few other parts and it was cheaper than buying locally.