I've said this before, there is no one size fits all when it comes to oil. Just buy the correct oil for each application and don't worry about having multiple oils on hand. Is it really that big of a deal to have more than one oil in your stash? Why does everyone want to pare it down to just one oil for everything? I'm not sure what that accomplishes.
It can accomplish simplicity, smaller inventory and potentially reduced cost. I don't advocate "one size fits all" for everyone, just me.
My Mazda 6 and Ford Focus have had a steady diet of xW-20 and are no worse for wear because of it. My Jag and E-250, as well as my lawn mower, generator, high pressure washer, wood chipper and snow blower, have always been filled with xW-30.
Even though the newer cars have done well, I don't think they have experienced any extra benefit from the "thinner" oil.
Even fuel economy appears to be no better with xW-20.
The EPA rating on the 2011 Focus, with automatic transmission, was 33 MPG. The best fuel economy I've ever gotten in the loaded (weight, not features) Focus is about 37 U.S. MPG. That was on a mountain trip, from Victoria to Edmonton. An average prairie run normally measures around 34 MPG, with 4 passengers and luggage (about 700ish lbs).
This past weekend I did a very cramped 450km camping trip, with 5 passengers, a jam packed 11 cubic foot roof box and trunk.
The Focus, filled with Pennzoil 0W-30 Euro LX, returned an average of 33.5 MPG.
For me, it just makes sense to use one, very good oil. Cheers