I use Quadra Synta semi synthetic 10/40 (from VW main agents in UK) in my GT850 Cali, 850 Le Mans, Golf GTI and almost everything else I come across.
It's cheap and very high spec.
Incidentally, I was told by Amadeo at Raceco UK that synthetics are a must in any air cooled engines as ordinary oil will cook as temps rise in traffic etc.