I've got a Y2K New Beetle with 2.0 liter and changed to Mobil 1 at 1000
miles.  Dealer did next change at 5K and I provided the Mobil 1 for that
change.  I told him that I had put in the synthetic at 1000 miles and
nothing was said about voiding warranty.  On a side note, it was raining the
day I wanted to change the oil, so I took it to an oil-change place (one of
the major chains) and they wouldn't touch it.  Said they had been told that
they couldn't service the cars at all while they were still under warranty.
Another oil change place did it with no problem.  Go figure.
Steve Sandlin
www.steveslockshop.com
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle