If you switch to synthetic in a high mileage car (100K+ miles), it may not be a bad idea to do a oil change a little sooner than usual in case the synthetic breaks loose some crud in your engine that plugs the filter. After that, you should be fine.
This is probably the best site out there to learn about oil and other automotive fluids:
http://www.bobistheoilguy.com/forums/ubbthreads.php