I own a 2000 Passat with the turbo motor. VW just sent me a notice that explained they were increasing my engine warranty to 8 years from the vehicle's original in-service date without a mileage limitation. The reason is engine failure related to sludge build-up. This is in engines that were following the manufacturers oil change interval of 5,000 miles. From this point onward the require the use of only synthetic oil that meet's VW's standard. They actually sent a list and not all synthetic oils are on it. They will be adding additional oils to the list.
This fits right in with the posts above. I had never used syn in my Passat. Even though I knew turbos could cook the oil and choke, accumulate sludge etc.,I thought that I had enough of a margin of safety because I change the oil every 5,000 miles. I don't run at high rpm in the heat under load except perhaps for a few seconds at a time once in a while.
Evidently VW had thought so as well, which is why the NEVER required synthetic oil. Until now. Engine failures out of warranty from good customers who were not using their engines outside of their design intentions. Note that VW has not reduced the oil change interval, and it has not said that if you use regular oil and change it every 3,000 miles. Regular oil is now outside the warranty. And those who did use regular oil and can show that they changed it every 5,000 miles have an eight year warranty on their pre- new oil standard purchased cars. Note that the car came new with regular oil and that is what VW dealers put in during regular oil changes.
So now I will switch. But my 1997 T&C van with 136,000 miles will continue to run on dyno oil.