I don't think I would be able to present a valid argument that CAFE isn't a major drive behind thinner oils in the USA. I say that it's not the only factor, but it certainly is a major one, and likely the most significant one.
Back in the day, the manufacturers listed a recommended oil for fuel economy; that was probably a regulatory requirement. I suppose the "requires" wording is the current regulatory requirement.
If CAFE doesn't actually require that the viscosity used during testing be listed as the required oil in the manual, then the alternative reasons aren't a lot better, really. One alternative is that automakers think we're idiots and will use 20w-50 in a North Dakota winter the second we get the chance. Or, they're so petrified of lawsuits and warranty claims based on such a misuse of a grade that they prohibit such oils altogether. Or, with longer OCIs, the automaker doesn't want us to feel obligated to do early OCIs just because of the weather, or doesn't trust us to do so. Or they don't trust lube techs to pay attention (they probably don't trust lube techs as it is an know they'll put 10w-30 in everything if they can get away with it) to putting the specified grade in for the ambient weather.
And to be blunt - and I'm not assuming that lighter oils sacrifice longevity; I'm not convinced of that at all - of course the automakers would look to satisfy CAFE first, assuming that a 20 wouldn't blow every engine up before warranty expired. While no automaker wants to be known as a producer of 100,000 mile and under vehicles, they have to balance that with penalties for not meeting fuel economy targets.
There are other ways to meet fuel economy targets, but the automakers have to pick what's most palatable. They're not just going to yank the F-150 from production or double its price as a disincentive to a lower fuel economy vehicle. They're not going to be producing a plethora of 80 hp stripped down vehicles, either. At current fuel prices (yes, they're higher, but still fairly reasonable), the market won't stand for such measures. Perhaps the automakers should tune every vehicle that gets under 40 mpg, for instance, to require premium fuel or E85. I don't know. I can't see the market tolerating that, either.
Remember that a brand new Audi, for instance, doesn't have a huge variation in viscosity choice either. While thicker (i.e. HTHS of 3.5 or greater under several OEM specs) oils are mandated, you don't have the option of running Rotella 15w-40 or PYB 20w-50 in there, either, at least from the standpoint of the manufacturer's recommendations.
I wish we had more choice. I know darn well that a vehicle specifying 0w-20 or 5w-20 isn't going to explode on 5w-30. But, Arizona isn't exactly littered with abandoned vehicles that failed on 0w-20, either. I don't know what the answer is, nor can I suggest a perfect solution.
