Oil viscosity - why the 20W-50 BS?

I too was raised up on the thinking that the thicker the oil the better it was. After seeing reports on how fast 40 weight oil flows at 20 degrees vs. 20 weight I saw things in a different light. It's entirely possible that an engine may need 20w50 oil if the clearances are loose but running thicker oil than you need is wasting energy and since it's cold flow rate is much slower it wears the engine faster. It all depends on the engine build.