20W-40 oil is the heaviest oil viscosity I have found yet in any Mopar FSM's of this era (but I don't have them all) and that is only for continuous temps above 32F . I would not be telling people to use 20W50 without knowing their climate and use patterns; Missouri gets waaay below freezing at times, and too heavy an oil will result in engine damage due to it not getting picked up and circulated properly at a cold start. This is even more likely to happen on an older engine that may have a worn oil pump. So, 20W-50 for the OP in this case would be a big mistake IMO. Maybe move up to 10W40, of a good grade oil.
My concern for the OP's case is that his pressure appears to have dropped as he is operating it. I would be thinking that the oil pump is well worn or the oil pressure relief valve in the pump is not closing right and allowing some pressure drop. It may also have the lighter pressure relief spring that limits pressure to 55 psi even with the pump rotors in perfect condition. I'd be replacing the oil pump as a precaution after seeing the cold and warm operating pressures drop across the broad.
BTW, I raced long distance races for years on Frams..... but am not sure I would use them now.