Which Oil Grade do I Need?

yeah, no harm in running 5W-30. You increase viscosity, but that in turn establishes a thicker layer of lubricant between your moving parts. You'll lose 1mpg or so, but that's no big deal in the grand scheme of things. If the manufacturers suggest running it at a GCWR of 14k or more, it's probably because of the load being placed on your crankshaft. Probably need a thicker/stronger layer of oil on there as maybe they've observed 5W-20 not being thick enough and occasional metal on metal contact at that viscosity and for those loads; Or it could also be that they want to ensure that a viscosity over 20 is maintained underload. What happens over time is that the oil additives that allow for multi-grade viscosities are "Sheered". So overtime as the oil is put on sheering faces, the oil thins. Since heavy duty applications tend to run higher RPM's and place more load on the metal surfaces, the engineers probably wanted to guarantee that an oil can withstand more sheering and remain above what the engineers deemed as the minimum viscosity to keep the engine protected. It's usually one of those two reasons. That's the usual thought behind picking a heavier grade of oil.

just a quick side note, going too thick of a grade can indeed cause some harm, like the oil not getting into the oil galleys fast enough due to thickness. But going from 20 weight to 30 weight isn't too big a leap and they probably designed the HEMI to use 30 weight oil originally.