I have just changed the oil & filter in my wife's 2.0L TDI Golf for the first time[second time for the car, first time for me].
Being new to these vehicles and knowing little about the oil filter system, I initially drained the oil via the sump plug and then removed the 32mm. cap on what turned out to be the oil cooler - I had expected to find the filter element inside the cooler - wrong. I then located the second 32mm. plug at the top of the filter/cooler and , presto, the filter element is in there. The thing that surprised me was that when I first removed the cap on the under side of the cooler a considerable amount of dirty oil poored out. This oil was trapped in the filter/cooler assembly and had not drained out via the sump plug. I can only assume that if one was to simply drain the oil via the sump and replace the element via the top 32mm. cap that maybe 200 to 300 ml. of dirty oil would remain trapped within the cooler assembly. This would be no different to diluting your new engine oil with an equivilant amount of used oil; hardly an ideal thing.
I am hoping someone will reply and tell me that I am wrong, otherwise I would be inclined to drain this dirty oil from the cooler at each oil/filter change.