[*] First, you want to put a reliable mechanical oil pressure on the engine and check the pressure cold and hot.
If the owner rebuilt the pump (kinda hard to figure out with the cheap replacement cost) and still had to use 50W motor oil, it tells you something.
I would drop the pan and remove a few crank/rod inserts and check the condition (plasti-gauge if possible). Inserts can be installed in-frame.
A high volume pump may mask the symptoms for awhile.