The pump will move a certain volume of oil depending on the speed of the motor. The pump only builds pressure if there is a restriction to flow. If the pump moved oil through the gears and just dumped it out into the oil pan there would be no pressure. The restriction to flow is mainly the tight clearance between the bearings and the journals. When an engine is new and has tight clearances there is not a large passage for the oil to flow and pressure builds from the restriction to the outflow of the pump. The pressure indicator fitting is in the oil filter adapter where the oil has pressure. As the engine wears the clearances get larger and the restriction is less, pressure drops. Building an engine with larger clearances will give you a larger oil wedge in the bearing journal space and also flow more oil through maybe allowing more heat transfer, but it will have lower oil pressure than a tight clearanced car especially at low to idle rpm. If the flat tappet camshaft gets scrubbed right away that material will be in the oil and wear the main and rod bearings which will result in low oil pressure and eventual engine failure.
That bearing material could be from incorrectly attaching the rods onto the crank with the fillets in the bearings facing towards each other rather than the radius on the journal, or incorrect clearances, or dirt, or a host of reasons, but the cause I've seen the most lately has been flat tappet cams wearing down. My buddies 455 only lasted a week or two until the oil pressure went south, the camshaft material was everywhere in that engine and ruined a crank to boot. That's the reason I went with a roller cam.
I have 25 psi at 500 rpm idle which I'm ok with, I'd rather have the 40 psi that region warrior has but I don't get that until she moves past 1000 rpm.
A lot of engines get built with larger clearances especially high performance and race engines but they spend most of their lives with the pedal matted at high rpms not much time at a red light in 98 degree weather.
