FWIW, I have been running 5w30 or 0w30 in my 5.7 with MDS for close to 50k now. MDS works flawlessly and all my UOAs show low wear for a Hemi. Several other guys on here have had the same experience with a 30wt oil. Don't be afraid of 5w30 in your 5.7 with MDS.
5w20 was specified for the 5.7 in the 1500s and cars to increase the CAFE numbers, not because of MDS.
Wish I could find the article I read this in, but it essentially said that Ford dropped 5w30 in favor of 5w20 in their modular V8s back in the late 90s because it gave the motors .5% better fuel economy number. In other parts of the world they stayed with 5w30. Here in the US the 5.4 3V is notorious for cam phaser/timing chain guide failures. Where 5w30 is being used they don't have nearly as many cam phaser/chain guide failures. I think it was Europe they were talking about but I'm positive.
Point is all of the big 3 had to do whatever they could to squeeze out as much fuel economy out of their vehicles as possible to appease the EPA. Think about it from their standpoint: drop a weight in oil to get .5% increase in fuel economy for hundreds of thousands of vehicles and the motors still mostly make it past the warranty period. .5% is nothing for the consumer but its HUGE for CAFE numbers.
The tradeoff of .5% fuel economy in exchange for a ridiculous amount of wear on your engine is not worth it. Plus, the only time a thin oil saves on fuel is at light engine loads, while idling, and at high RPM (has to be light load). Basically, all the situations that you won't be in 99% of the time. I mean, who's sitting still, idling their engine, or who is traveling at highway speeds in low gear?