If you are not leaking oil now, you won't with a good synthetic.
Synthetics don't cause leaks, but they do expose them. They get rid of sludge and grime that could have been preventing a leak from leaking as much, and then you got a worse bigger leak on your hands. This getting rid of sludge and grime could be good in some cases, but in most cases, this is just a problem that could have been prevented by just sticking to your regular oil, and could sometimes even ruin a perfectly good (but sludgy and grimy) engine.