does dino oil make the gaskets swell (thereby improving the seal), while synthetic does not make them swell? if so, will switching to synthetic result in the gaskets going back to their original size, increasing the likeliness of leaking?
I've never seen a car leak more because it was switched to synthetic. It's possible because the oil is more viscous and can sneak through smaller gaps in the seals. It really doesn't have anything to do with the seals swelling. To be honest with you, I would rather have a small oil leak, then put something in the oil that might hurt the longevity of the hard parts like the rings and bearings. If you put that oil thickener in there, it's going to keep the oil from circulating as well, and reduce oil flow to the the critical parts in the valve train. Synthetic oil will help the longevity of your hard parts, and if it leaks a little oil, it's worth it IMHO.
Another thing about those stop leak bottles of crap you put in your engine. They stop oil leaks by causing the seals to swell, which actually makes the wear out even faster. It also thickens your oil and causes more wear to hard parts. If you have a 1985 Dodge Caravan with a massive rear main seal leak that you are trying to sell, it's probably worth it. But I wouldn't put that stuff in a car you actually care about.