The oil the dealers use is Castrol SLX Professional Powerflow. It retails for around $80 if you can still find it. My local VW dealer had it in their spare parts store and I bought it from there once but last time I was there they didnt have any so they recommended I use Castrol Sport Edge 5- 30w which I have been using ever since.

I know they use this oil because it said so on the receipt when I got my car serviced. The dealer charge around $100 in their service for the oil.