A high horsepower direct injection turbocharged engine with a history of some fuel dilution in its past and the decision is to not follow the manufacturer recommendation on oil viscosity... and to go thinner to boot. As noted, this engine series did start with an xw20 recommendation. Yet ford reversed that. Interesting thought process...
As far as the premium fuel discussion goes, the only reason the EcoBoost engines can take advantage of higher octane fuels is specifically because they were tuned that way,and as result can use less boost with the higher octane. Even scientific testing picks up on that. (See the NHTSA report on the 3.5l EcoBoost as an example).
What that does not do is improve cost per mile metrics. The cost for premium fuels is greater than the fuel mileage gains achieved.
And while it's easy to say just make it all premium, it won't cost more, it isn't that easy. Refiners have only so much capacity to refine high octane fuel. There are limitations based on feedstocks, refinery equipment, and blending capabilities. The data I could find on short notice indicates less than 10% of fuel sold today is premium. Does anyone think that making the other 90% premium is as simple as the refiners just flipping a switch? It isn't- and the end user would need to pay that cost.