Waggy,

Welcome to the Stovebolt. I'm not sure I understand your last post. You need to put a known good mechanical gauge on the system to get a bullet proof reading of your oil pressure...you want to bypass any questions with a sending unit or electrical gauge.

If your mechanical gauge verifies the cold pressure at 18lbs, you have a problem. It should be much much more than this, and with a new engine, should drop very little when warm.

If all your rebuild clearances were proper then the assumptions would be; the pump isn't picking the oil up or there is a massive internal leak somewhere. I don't think this much pressure/volume would be lost in the lifter area without some other telltale rattling or banging. You said you changed oil pumps...did you remove a new one and reinstall another new one. Do they come with the pickup tube installed? I don't know if a loose pickup could draw air and result in diminished oil flow.

Oil that has been thinned by fuel from a leaking fuel pump won't develop as much pressure as good oil...does your oil smell of gas or seem extremely thin?

Stuart