Magnum motors will be no different than any LA engine for the sake of this discussion so your Magnum experience is valid. As far as running 20w50 in a small mopar or any engine in a warm climate, you will not hurt anything. Engines with with variable valve timing like the 3.6 pentastar or multi displacement such as the late model hemp require the thin oil to operate their solenoids and cam phasers. They will not be damaged by thicker oil they just won't function correctly and will set check engine lights.
To the OP, remove the oil pan and inspect your bottom end. I find it easier to remove the engine to do this on anold A body. I suspect you will find a spun rod bearing. Good Luck.