2 issues not mentioned, sticking pressure relief valve in the oil pump, air leak in the pick up tube connection (block pipe thread, or pickup screen). Oil pressure seems erratic. Remove the pump, check the relief valve. Even a fleck of metal will stick a relief valve, DO NOT radius the cut outs, they need to be sharp, minor scoring can cleaned up with Crocus cloth and kerosen. Idleally 20 psi at 750 rpm at idle, 10 psi acceptable (20 better) 10 psi per 1000 rpm are what these engines were designed for. Max oil temp, figure 250F on synthetic, 15W-50, peak oil temp is 50F over pan temp, so 300F. 180F thermostat and a good radiator, and good water pump will keep condensate out of the oil. Ideal oil temp is between 180-220F.
Hope that helps.