The difference in drag at 12 viscosity versus 9 viscosity at the wheel would be near non existent. FCA even changed spec for the eco diesel for widespread damage caused by thin oil, their fix, go up in weight, why?? Why would that be their fix? Because the **** is too thin.
Now, why not "fix" our hemi's by changing the spec like they did in another application? Because they can't, they have sold you a bill of goods that mds requires 20 weight oil. Now, there is widespread cam and lifters issues that has never been seen before. What was their "fix" for non mds, make the 6.4 use a 40 weight oil full of molybdenum. Well geez, why go thick in eco diesel and non mds hemi but go water thin on mds hemi?
Literally the only thing against running 30 weight is piston slap in cold places. You can opt for 0w30 in those places, but whatever. Learn all of the info and make an informed decision. If you are a "Woke" activist and all up in the gov't agenda, go for it run the thinnest stuff possible, I support that 100. If you have evaluated the problems that plague hemi's and believe the manu has let us down, there is like one move for you, go heavier with the oil choice and perhaps choose an oil that is built out of the government agenda api class.