Do a search on PVC valve. VXcaver was experiencing a big oil loss and found it to be the PVC valve. After changing it he has not had a problem with oil consumption. He did a write up on it.
Sounds like your local Zu dealer is looking for time. The type of oil/filter you are using is better than what they want to put in it and should have no bearing on the problem. Instead they should be using their time trying to figure out where your oil is going!