Rod Knox, I can’t speak to what Lexus or Willys did, but if it’s possible to spec a single grade of oil for all operating conditions then it’s a good thing to do to reduce customer confusion. And there’ve been a lot of changes in oil chemistry and engine design practices in the last 60 years.
Docnick, yes, fuel economy regs are one of the things that drive viscosity, but they aren’t the only thing. Sure, a manufacturer might recommend different viscosities in different climates, but before they do they validate the engines that way. The fact that some vehicle have multiple oils spec’d doesn’t mean that it’s a good idea for the owner of a different vehicle to use an off-spec oil. Also, if you live in a severely cold climate and thought that you’d increase the sub-zero cold start viscosity by going from 0W-20 to 0W-30, you may well have actually ended up doing the exact opposite of your intent.
the same mountainbike, yes, in a nutshell that’s how the friction modifiers work, but their effect on high-temperature-high-shear viscosity (which dominates in the hydrodynamic bearing gap) is different than their effect on the low-temperature-low-shear viscosity (which dominates in most other places). It’s one of the dozens of engineering tradeoffs that need to be comprehended in the design of the engine.