I see it all the time at my VW part department, would have thought Audi use the same. Its also a bit cheaper, £25 for 5 litres. I asked just to make sure it wasn't a crappy oil, since most people here seam to use Castrol.
If its much better then I might be swayed to use Castrol, but if Quantum Synta Platinum is sold at the dealers and fairly cheap i dont see why people would need to buy more expensive oil?