I build Opel CIH engines and there is a school of thought for 20W50. It stems from 2 things:
- Running at 150 kmph on the Autobahn, 20W50 is recommended for the high temps.. that is like an endurance racing situation
- Guys with worn engines will use it to keep the oil light from coming on in central CA in summer temps of 100F at idle
So some think 20W50 is going to be great in Canada in winter LOL. What they don't realize is that the only oiling to the lifters in that head is down past the lifter bores....there is no splash oiling. No way 20W50 is going to flow as well there as a 10W30 or 10W40. And so they get lifter wear and patch that up by putting an oil 'dam' in the back of the head around the drain hole to make the oil pool up in the head. But that would not be needed with a proper oil weight.
So yes.. viscosity and flow do matter in the real world. It is nonsense to say otherwise.