I wouldn't say that it's impossible for oil leaks to develop due to switching to a synthetic oil, but it wouldn't be due to the viscosity.
Synthetic oils containing PAO can in theory cause problems with seals, though they should contain seal conditioners to counteract this. In any case, PP and PUP don't contain much if any PAO.
When switching oils, it's always possible that the new oil with have worse seal compatibility for a given engine, and if a leak develops, you'll hear complaints about the new oil. It's also possible to switch to an oil with better seal compatibility, but you're unlikely to hear stories about these cases. This doesn't have anything to do with whether the oils are conventional or synthetic. Switching to PP could make leaks more likely, less likely, or it could make no difference.