“Do not use oils of a higher quality than specified”

A same grade synthetic oil will not provide more protection. It will resist breakdown and can be made n the sumo for longer, but engine protection remains the same.

that scooter engine is like an old sawing machine, it has to do WOT, otherwise you’re not moving.

OP just has a vivid imagination. He imagines bad things and how a synthetic will solve them. Where his usage is not different from thousands of others in much hotter climates like India.

I go to Canadian tire and pick the cheapest diesel 15w40 oil I could find and run it. Synthetic is a waste in this application IMO.
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You can't go wrong with Yamaha own synthetic YamaLube 10w40...

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I'm more worried about having enough Minimum Oil Film Thickness left to keep the bearings separated. Most air-cooled Chinese scoots love to seize up in the summer, so this will be my precautionary measure.
It's the pistons that seize in the bores, not the bearings.
Old fashioned thumpers have very large piston clearance because of that phenomenon (note Enfield in sig).
Usually after a cool down they kick right over again.
My 1992 fatboy calls for 20w/50 SG/CD oil for the engine (iirc)

Although the sae spec is obsolete (well probably superceeded) it is a pretty robust package and i have no trouble using it in anything more modern, but following manufacturers viscosity requirements

CD had a long run, 1955 - 1985, when it was obsoleted by CD 11 if my books are correct
Some confusion about "CD" which is an API from 1955 and obsolete.
"CF" Obsolete from around 1994

Valvoline sells a Motorcycle specific 10/40 MA conventional oil the other motorcycle oils in here will be fine too.