While I agree that synthetic oil doesn't cause leaks, I also don't think that the majority of leaks, that are identified with a switch to synthetic oil, existed prior to the change. From this point of view, if you find leaks after switching to synthetic you simply caused needless repairs, especially if there isn't a real justification for the added protection that synthetics can provide.
Example, my 85 Toyota truck had the classic 22R motor in it. When I went to synthetic oil I developed some significant leaks from the valve cover gasket. I quickly switched back and things went back to normal after a short period of time. About a year later I tried a blend and it started leaking again, this time out the opposite side of the cover. Again, back to conventional and it stopped again. The motor was running strong when I got rid of it. Stupidly, the guy that got it wanted EFI and swapped out the motor and couldn't get it running so he put the old motor back in and had difficulty getting it running again. Moral of the story, if it isn't broke don't fix it.
