Depends on the type of Synthetic it seems. If its a good one like M1 or PP, they don't seem to do it. I can attest to it on 4 older cars. No leaks.
But also, if you've got some miles on your vehicle, what is the problem with just changing some gaskets?
As far as I've seen, most cars will start to weep a bit of oil at 100k+, synthetic or not, I think its just that people start to pay more attention to leakage once they've gotten the idea into their head that it will cause more leaks to happen.
I've made it a habit to change gaskets where common shrinkage happens, such as valve cover gaskets and grommets, the kit is maybe $20, and rather simple.
As far as switching in winter and summer, that makes absolutely no sense at all whatsoever. Synthetic is an advantage just as much, if not more, in the heat, as it is in the cold.
I personally suggest choosing a GOOD synthetic, such as M1 or PP, and to change any gaskets that are leaking. Also, be sure to keep a clean engine so that you can monitor any leakage that occurs.