If you convert to synthetic on a high mileage vehicle, expect oil leaks and to start burning a little oil. I have discovered that issue on 3 different vehicles. That's the better detergent additives doing their magic and scrubbing through carbon build up keeping the gaskets and seals from leaking.
That being said, synthetic is superior oil, through and through, especially the more expensive stuff. I run Amsoil in my truck. Ive seen tests and experienced the wonders. I run it in my '97 K3500 5.7 in the engine, t-case, and differentials. Things generally do run smoother and my oil pressure even went up.
In general, though, a good oil is not effective without a good filter. I use the Wix XP filters, because I love Wix products, but any Wix, Mobil 1, Amsoil, ACDelco, Motorcraft, Mopar, etc oil filter will do just fine. Stay away from cheap filters (*cough* Fram) or the ones used at quick lube places. They get away with being cheap because the filter media is garbage and sometimes they don't have a drainback valve or pressure valve.
If you're adamant on your engine lasting as long as possible, use Amsoil and an Amsoil or WIX XP filter. If you just want your engine to be kept up, just get a decent oil and use a good filter at 3-5k intervals.