Seems excessive.....do you have any oil leaks or excessive smoking from the exhaust? I've got no experience with the diesel motors but on the gas ones, as per your mechanic I do believe that VW has set quite a high bar for what amount of oil usage is considered 'normal' - but that being said, my reality with my two cars (even the high mile one approaching 150k miles on the clock) is that they burn very little oil indeed; I've personally never had to top off between oil changes at 5k miles intervals.