The 5.7 HEMI with an automatic always required a 5-20W oil for the MDS to operate properly. The newer lighter viscosity spec, of 0-20W reduces internal viscous drag and allows for quick, seamless valve timing changes and MDS hydraulic actuator switching. A 0W-20 oil and a 5W-20 oils has the same viscosity when the oil reaches the operating temperature. Here's a good article, though rather technical, regarding viscosity. What you'll find is that the real difference between an 0W and a 5W is in two areas- Low Temp Cranking Viscosity and Low Temp Pumping Viscosity. See the first table. Putting the Simple Back into Viscosity - Bob is the Oil Guy
The FE difference is in the 0.5% range and probably can only be measured by exacting laboratory engine tests. The cranking and pumping difference is 5°C (9°F) so is small. That is also measured at -30 and -35°C, so essentially very few people will ever start their cars at those temperatures. And if you do live in climates that get that cold, you already know about the difference. The rest of us couldn't be bothered.