If your TDI is a PD type then you're better off erring on the short/fast side. There were many cases of prematurely worn out cams on PDs due to oil.
This means sticking to the oil norm and change interval specified by VW.
If the engine is a non-PD TDI (such as ALH, AFN etc.) then you could try using oil analyzer (such as lubricheck) in order to determine the level of oil wearing. This can be done at the end of the usage cycle or even during use.
For example, in my common rail TDI I used to change the oil at 15.000 km but upon analyzing the oil quality I am going to bump this up to 20.000 km (90% city driving but at unusual hours - almost no stop-and-go traffic).
In a TDI, in general, its advisable to not cut costs on oil and fuel. Use the best that you can find, it's better and, in the long run, it's cheaper.