Suspect worn engine bearings. An oil pressure test with a mechanical gauge will verify the pressure really is low. Debris could be blocking the oil pickup too but the knocking indicates bearings. You can try a little higher viscosity oil. If worn bearings are the cause, the pressure will remain higher longer. If debris in the oil pan is the culprit, I suspect the thicker oil will have a harder time passing by it and the pressure will drop sooner.