Well...I did say "possibility" of fuel dilution. Surf some on BITOG and you'll find that turbocharged di engines have the reputation of diluting the oil with fuel. One explanation/theory I've heard is that with atomized fuel immediately present in the combustion chamber, the very high mean effective pressures in the turbo-charged cylinder can push minute amounts of fuel past the rings. I don't know if it is technically true or not and I'm not prepared to argue the point. I have read that if fuel dilution gets above 3% you've got problems (mainly I think with viscosity reduction but also adverse effects on additive performance). I just did a used oil analysis (UOA) at only 1500 mi on my N54 on the factory fill and there is some very minor fuel dilution noted (less than 0.5%), and the averages (as noted by the UOA lab) for these engines can get up to around 2%. I've also read that fuel dilution is fairly common in diesel engines - - don't know why exactly - - but that for some reason diesel oils can tolerate as much as 6 or 7% dilution (probably the nature of the fuel).
If you're not familiar with BITOG (Bob Is The Oil Guy), it is a forum of about 35k members obsessed with lubricants. Most are rank amateurs (like myself) but there are some very knowledgeable people who are professionals of one stripe or another who work in the lubricants industry. I'll post your question/challenge and see what comes back. In the meantime, here is the BITOG forum on euro cars:
http://www.bobistheoilguy.com/forums...oard=48&page=1 There are a couple of threads going on now on the N54/N55.
I'm running German Castrol in my motor now and will do a UOA after around 2500-3000 mi to see what I get. I'll post the results. If everything looks ok, I'll run longer oil change intervals. The z4 is my first turbocharged di engine.