As good as new petroleum based motor oils (SL rated) are, synthetics are better at their job. Period. Are they worth the extra cost? Depends on the situation. Are they better at lubricating at extremes temperatures? Yes,far superior.

Synthetics keep your engine far cleaner, let it run cooler, and protect it better during cold starts and high temperatures.

As far as running synthetics longer drain intervals, I wouldn't. Two reasons. 1) If your vehicle is under warranty, you MUST change your oil at the designated intervals, to keep your warranty intact. 2) While the oil is still capable of lubricating, it will have contaminates that the oil filter is not capable of removing, such as fuel and microfine dirt and carbon particles. The only way to remove them from your engine, is to remove the oil, no matter what kind of oil you use.

As far as leaks and rings not seating because of synthetics, I can tell you that General Motors uses Mobil 1 full synthetic as a factory fill on all Corvettes, SS Camaros and GMC Denali SUVs, with no ring seating or seal problems. They use synthetics in these high perfomance vehichles, because it is better. If you own one, you must use Mobil 1 or equivalant to keep your warranty intact. The only oils that are equivalant are all synthetics. No standard petroleum based oil meets the GM designation.

The best thing you can do is change your oil and filter regularly and use the proper viscosity oil. I would also suggest you use a factory oil filter such as AC/Delco for General Motors, Motorcraft for Ford and Mopar for Dodge. Cut a Fram filter apart sometime and you will see why.
Hope this helps.