Two service related items

When I bought my Passat (97 GLX VR6) my VW mechanic (not affiliated with any VW dealer, just works on VWs) told me that using synthetic oil is best when I change it. For the past two years, I've always used synthetic and have been getting it changed regularly about every 4500 miles or so. I've heard now that synthetic doesn't need to be changed that often, is this true? If so, how far is reasonable to go between changes?

Next item, my timing belt. There are horizontal lines in it, which I thought was a sign of wear. My mechanic told me that it is natural for a timing belt to develope a lot of lines in it. The car is just breaking 100k miles, and I've read a lot about these things breaking, especially with newer Passats and Audis. Is this a problem with the newer cars and not indicative of mine? Should I have it changed regardless? Is there anyway to tell when it's time to change it? What should I expect to pay for such a job? Are there any other service items I should be looking into as I break the 100k marker? Any information would be appreciated.

VR6's don't have timing belts, are you talking about the serpentine accessory belt?

Thanks Boschmann, I didn't realize there was a difference between a timing belt and the serpentine accessory belt. As you can tell I know very little about cars in general, let alone VWs. It's the only large visible belt, so that must be it. Do the same rules apply? Should it be changed?

Hopefully it's already been changed as I've never seen one last 100K. There shouldn't be any cracks on the ribbed side of the belt.