RE: Synthetic oils in a 1984 5000s
Any clues? Yeah, probably the bearings or the oil pump is shot. Hook up a cheap mechanical gauge just to make sure that it is not the pressure sensor. I think that the previous owner put in 20W50 or some additive (like motor honey) to build up oil pressure and quiet down noisy bearings. When you drained the oil you replaced it with a thinner synthetic which is easier to pump and lowers the pressure. There is also a pressure regulator mounted on the pump and is easy to access, maybe the spring broke or weakened.