Sunth-oils do a much better job at cleaning dirty engines and keeping clean ones clean. Usually there is no reason to run any type of oil/crankcase cleaner before the changeover. Where syth-oils take a bad rap is when it starts cleaning the dirt in an engine that is clogging up oil leaks. Most oil leaks are easy to fix. There is no reason to not change over to synth oils at almost any mileage, except on initial break in. I just purchased a new '04 Turbo Baja and as soon as I hit 7,500 miles (maybe 5,000) I'm going to synthetic oil (RedLine! or Royal Purple are my favorites) and stock oil filters until I know I have found something better.
I've seen MANY Corvette engines pulled down that have always used syth oils (usually Mobile One), and in almost every case, you could still see the original factory hash marks in the cylinder walls. Usually the bearing were in great shape. No significant wear at all internally. (These engines were being pulled down for performance enhancing rebuilds)
By all means, if you want to go to synth oil, do so! I always keep my oil change intervals at 7,500 miles nominally.
I use syth oils in all my cars, in all areas (i.e.- tranny, engine, p/s, etc.) and have never looked back.