That right there. A single-grade at high temp is thin, and will run off just as fast as the multigrade. That engine stays hot enough to let anything off that wants off, but even then there is still a very thin layer left on it. The cylinders and cam and lifters and gears, for instance, do not get completely dry. "But," some will say, "why the corrosion on those parts?" It's because combustion blowby contains water vapor, and water vapor condenses in that crankcase once it cools off, and water forming on parts with oil on them catalyzes (due to the metal) with the oil to form acids that eat the metal. Electrolysis and acids. Short flights, or ground-running, is a real good way to shorten the life of the engine. It doesn't get warm enough to drive that accumulated moisture out of the breather.
A thicker oil won't help that. The thing that prevents it is storage oil. It has corrosion inhibitors in it.
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https://www.shell.com/business-cust...da955d588ce5/theaeroshellbook-edition2021.pdf
https://www.shell.com/business-cust...raulic-fluid/fluids-and-preservatives-2f.html
Pilots and owners can believe anything they like, but many of them believe popular aviation myths without knowing it. There are plenty of authoritative sources out there that give accurate information, and older, experienced mechanics have seen the damage caused by the stuff non-mechanics want to believe. It's like refusing to listen to the doctor when he tells you to stop smoking and lose weight or else.