I highly doubt that simply switching to a synthetic oil would cause you car to start to leak. Only if you put a thinner oil in would I believe that it could happen. But don't take my word on it! I haven't been around for too long yet!
I've heard that certain synthetics do not have the detergent (keeps the engine crud in solution) properties of good ol' natural oil, and so tend to leave deposits of engine crud in your oilpan. Ideally, you want an oil that doesn't allow the crud to settle-- this way you can get rid of it when you change your oil.
As for which brand to use? I use Pennzoil Full Synthetic, but only because I used regular Pennzoil for years in my Taurus Wagon. I've read good things abour Pennzoil.
