You guys are saying your engines wouldn't make the miles unless syn oil is used?
Maybe not if it's direct injected, or a turbocharged application, or simply driven very hard.
Also, let's say the engine does make it to 300k either way, but if you'd run a good synthetic the internals of the engine are much cleaner and the engine is still running perfect and can keep going well beyond 300k, but the one running conventional oil is on it's last legs and is burning oil, idling terrible, getting poor MPG and can't even pass an emission test anymore.
IMO, your best chance of having an engine make it to 300k and beyond AND still be in perfect running order is to give it a premium synthetic oil.