I would agree that the correct oil to use is Mobil 424. I have used this oil in a Kayaba japanese p/s pump and is as quiet as a mouse. Don't let any air into the system, but if you do need to refill it after air has entered the system, jack the front end off the ground and crank it over whilst turning the steering from side to side. If you just refill it after air has entered the system and use the idling engine to pump it around, the air contaminated oil will just circulate through the system leading to annoying pump groan on steering application.