I will mention that when Ford introduced their modular engines in late 90s, they were at 5W-30. Then in year 2000, they switched to 5W-20. They even back specced previously built motors to 5W-20 even though they are no longer counted with CAFE.
Their reasoning was that the thinner oil, while does improve mpg, also lubricated the modular line of motors better, so they recommended it to previously built motors. Not sure if this applies to the Hemi, but it could I suppose. But while I'm under warranty, my motivation is to stay with 5W-20W, for now at least unless I find info to the contrary.
Even though our Toyota 2.0L & new Suburban 6.2L both recommend 0W-20, I'm not married to that oil if it's proven to be only for fuel economy at the expense of durability. The search continues