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

18,000 hours of gasoline could have bought and paid for an adequate solar setup, no oil needed.

As far as oil- my first truck used non detergent 30 weight, no oil filter. Hauled tons of hay. oats and gravel, 250,000+ miles. Flathead 6.
My last tow vehicle I would have to start in sub zero weather then pull the full rated gcvw. Synthetic was the *only* way to go for that. For a small air cooled engine- meh.