High quality modern diesel engines do not have the need for freqent oil changes .Tollerances are much tighter , diesel oils are MUCH better than 20 years ago,Oil filtration and circulating is much better than years ago , fuel injection cut of the fuel first so there is no chance of fuel leaking past rings to contaminate the engine oil , diesel fuel dose have a better lubrication property than gasoline . THe design is for long lasting engines so bearing etc are bigger /stronger .They will happly run for 500,000 miles so at 20,000 miles a year thats 25 years of use . My Mercedes sprinter 5cylinder turbo calls for oil changes about every 12,000miles but a lot of people run them for 20,000 plus miles . Mine holds 9.5 litres of oil so check the capacity of your VW .