Hi there,
I don't know whether it's any help but I recently had my 20v T serviced @ Awsome GTi in Manchester.
I wanted MOBIL 1 fully synthetic oil on the basis it was the best oil available. I was advised not to use it due to the mileage of the car (100k). Makes a change a garage actually wanting to a) save you money and b) looking after your motor at the same time!
Why not give them a quick buzz on 0161 776 0777 and just ask the question. They won't muck you about and will give you free advice. Just let them know the age/mileage of your GTi and i'm sure the'll take car of you.
Cheers
Ben