Synthetic oil worth it? 18k hour yamaha generator says no.

I overheated an engine in a '78 Datsun. The dino oil would have burned to it's carbon beginnings. The synthetic just ran out when I drained it. New engine vs. synthetic? Been using it since '78.
Traded my '90 Suburban at 228,000 mi...wasn't using oil.
Traded my '98 Suburban at 183,000 mi...same result.
I'm sold.