James, I suspect we are saying the same thing in different ways. I have been an operator working in the petrochem industry for 33+ years. Many classes in fluid dynamics, including an associate degree

You must agree that you can have a positive displacement pump moving its full capacity with no differential required. Centrifugal pumps must operate inside it's "curve", but that doesn't apply here. Assuming no cavitation, a PD pump will move its capacity SOMEWHERE. Back pressure just forces the fluid to escape anywhere it can. You can have 200 psi at your oil pressure gauge, but if it isn't flowing where you want, bad stuff happens.

That was what WJ was writing about in the article.

RIP