Ok, I could give you a long answer with history etc. but I'll spare you and give you the short answer. Yes and No.
No - synthetic oil will not make you seals leak.
Yes - if you have seals which are going bad already and sludge from your old oil is helping plug potential leaks the synthetic oil may clear out that sludge and so a leak may appear but that is a bad seal and not the synthetic oil creating a leak.
Originally Posted by Quiet Lunatic
Looks like you went ahead and did it. I was gonna suggest that you install the proper weight and also do a flush do there is no DINO oil residual left over to mix with the new Synthetic Oil.
No reason to do that Dino and synthetic are compatible. However, if you suspect there is a lot of sludge in your engine I would not go more than 3000 miles before the next oil change. Save extended change intervals for a clean engine.