@chilehed Your statement is generally true, but many oil specs by car manufacturers are partially driven by CAFE requirements, and using a thinner oil will squeeze that extra fraction of a MPG on the test, at the expense of long term engine life. My Toyota book says to use 5W30, but two years later I got a bulletin to use 5W20 mineral base or 0W20 SYNTHETIC.
My son's Maxda3 says to use 5W20, but this same car in other countries will use anywhere from 5W30 to 10W50 (not recommended) in some tropical countries. My son diregarded the book and uses 0W30 full synthetic and after 110,000 miles of hard driving, some of it in the SW desert, the car consumes virtually no oil between changes. No European car manufacturer specifies 0W20; that would not work at 100 MPH on the Autobahn.
Oil recommendations are of necessity a compromise between minimizing friction (and maximizing EPA mileage) and preventing undue wear. My wife's new Mazda specifies 0W20 synthetic, and the dealer will not put in anything else. I would be OK with a 0W30 (we have cold winters here), since cold start engine wear can be severe. However, my wife will not race across Arizona in July.
I agree with you that it is unwise to put in an oil that's too heavy and won't allow the valve gear to be operated properly.