Everyone has an opinion on this so you wont get conclusive advice here. My tupence worth - the VW recommended oil for 1.8T engines is 5W30 fully synthetic, I have yet to read compelling enough evidence to go for anything other than that. Whatever you go for though make sure its good quality stuff, Castrol / Mobil / Millers / Quantum etc and not supermarket or Halfords specials, I think that would make a far bigger difference than going to 5w40 or 10w40 etc.