I had some Valvoline 5w-30 in my turbo engine when I tried blowing it up. I blocked the blower housing inlet, but the engine just wouldn't bog or blow. I drained the oil and it came out like black water. After that it scattered no problem, but that's irrelevant.
An extreme example, but that's where synthetic really starts showing benefits. In anything but a race-prepped or badly abused engine, the cost:benefit ratio isn't that great. I've had petroleum oil do the same thing above, FWIW.