I also do mine in between services every 6 months. Still get services done for the history and stamps really
It's good you change it every 6 months yourself but this isn't saving you any money of you and it to the stealers for an annual service. Unless its a big service (40,000) they aren't going to do much else other than change the oil.
The amount of times I hear people saying they do it for the dealers stamps is scary.
Dealers stamps count or nothing in my opinion. Seeing the VW stamp actually puts me off.
Most VW dealers are a bunch of ass clowns. Well the ones I have dealt with are, and not to count all the horrendous stories on here of how shocking VW service is.