I made the switch at 80,000 miles to Mobil 1. Sure enough, I developed a slight oil leak, which really irritated me since it hadn't had any before. For whatever reason ("blind hog finds accorn") I kept using Mobil 1. After about 10,000 miles the leak went away and stayed away. I use virtually no oil between 5000 mile changes, my car starts MUCH easier in below-zero weather, and my oil pressure is higher when idling in Midwestern summers stuck in traffic with the AC on.
I have read elsewhere that, in keeping with Mr. Swinford's (hope I spelled it right!) comments above that the superior detergent properties of synthetics SLOWLY dissolve years of accumulated gunk all over the engine, and that in some cases leaks develop temporarily since the rings may get "unstuck" from their grooves and actually increase blowby FOR A WHILE but then after the rings reseat themselves all is better than before.
Don't know if that's absolutely true, but I would say my mileage has increased from 20 mpg in town to 23 just recently, 20,000 miles after the switch . . .