My 2 cents: Synthetic blows conventional out of the water, the sooner you use it (after break-in) the better off you are for sure. Too many wifes tales still floating around about synthetic. I use Amsoil which has extened drains of 25k miles for the oil and 1/2 that for the filter. I also use filtermag.

Now if you have a leaky bucket of bolts and switch to synthetic then yes it will probably leak even worse, but if your engine is in good shape then you should have no problems. At 140k miles on my '93 DOHC Grand Am I switched to amsoil. I did a oil change of 1/2 amsoil and 1/2 conventional for 1000 miles then switched to all amsoil. The 1/2 & 1/2 was to slowly clean out any varnish. I did this a year ago and have had no problems. I wish I had switch a long time ago. Modern chemistry is a powerful thing.