Therein lies the problem with enduser testing. It is a leap of faith of sorts. Why does the mfr specify what they specify? Not only because it's widely availble, but because they have put countless hours into testing it. So when we step outside the lines, we have to make sure we don't step TOO far outside the lines, because we simply do not have the means to perform that same level of testing. Maybe there isn't any significant damage by using a 5W40, or a 0W40 Euro Spec oil in Ecoboost engines. But how are we going to prove it out? Scientific field testing is not going to happen for the vast majority of people.
BUT, we CAN compare oils based on their OVERALL specifications, which is where the ASTM standards come into play. When we look to alter the fluid we choose to go with, we are looking for: better Noack, better HT/HS, better flashpoint etc or any combo of those things. Viscosity is kind of a basic requirement, followed by the additives. Given proper viscosity, will the additives cause harm to the system over a long period of time? Why is ZDDP no longer popular, why is boron used instead? There are legal aspects of course, but engine design has also been altered to run better with modern additives. Lower operating temperatures, tighter clearances, altered seal design, combustion & deposit characteristics, etc.
Similarly transmission fluid, coolant, & gear lube. Many choices ...
BTW, AnotherGreenFusion had had some Blackstone Analysis done on virgin engine oils, if you are interested. That post is somewhere around here.
http://www.ecoboostperformanceforum.com/index.php/topic,6907.msg106169.html#msg106169