I'm not a mechanic, nor do I play one on TV, but I know some well respected mechanics and trust what they say. (One of those has posted in this thread).
Of all the vehicles I've owned, the first (and only) vehicle I ever used fully synthetic oil in is my truck. I've only done that because the previous owner had switched to synthetic at the first oil change.
Our Volvo which has 180k miles came from the factory will a semi-synthetic oil.
Mrs. Clean's VW Beetle TurboS uses regular crude and has @ 110k.
All of the above are serviced at 5k intervals.
All well known national trucking concern has their R&D division perform a cost/benefit analysis on the use of synthetic vs non-synthetic oils in their substantial fleet and found no cost benefit for switching to synthetic. It is worth noting their equipment and driving conditions are not similar to that of the commuter. They also go far beyond 5k service intervals.
Re changing over from non-synthetic to synthetic: According to my sources, this can present problems such as creating leaks where there were none.
And as well stated above, use an oil which meets (or exceeds) the specifications required by your vehicle's manufacturer and follow the specified maintenance schedule and you should enjoy many happy miles.