While various manufacturers of synthetic lubricants have done dyno test and have shown about 3 - 7 hp gains (depending on engine) from switching to synthetics, I would not switch over to a synthetic motor oil solely on that premise.
To answer your question about seals and gaskets, no, there are none in modern engines that are incompatible. I think you'd have to go back to the early 70's or so before you'd find any engines that had those kinds of issues and thats assuming they are still running OEM gaskets and seals.
You'll hear reports also that switching from conventional to synthetic and not being able to switch back and vice versa, and those are all nonsense. You can switch back and forth to your hearts content.
Hopefully that info will help you out some.
