Oil viscosity spreads..
I wonder if we're starting to sound like obsessive/compulsive people when it comes to our cars...
The way I have always understood the viscosity issue, at least with regard to "dino" oil, is that the greater the spread the more modifiers they formulated into the oil. Apparently, the oil can only be produced at the lower viscosity and have oil thickening modifiers added to achieve the flow characteristics of the higher rating.
Why was this undesirable? The thinking is that these modifiers have higher specific heat content characteristics than "unmodified" oil and thereby cause the engine to run slightly hotter. As we all know, more heat, more breakdown of lubricating molecules and hence more wear.
I do however believe most of these arguments go away with a good synthetic, but I still avoid running x-50wt if it's not needed. In my climate, it's not needed.