I always thought that you'd use loads of the stuff if it's that thin. As Rich says, don't compromise on oil, use what VW recommend even if it's their own Quantum Syntra gold. I always use Quantum syntra silver from VW in mine and it seems good to me, I use a bit between oil changes and services so have to top ot up a little so I can only guess at how much of the thinner stuff you'd use.