I guess the thing that I don't understand is that IF there were excess clearance or a internal oil leak somewhere, wouldn't I be seeing a drop in pressure across the board? Like I said the only time it dips to 5psi is after the engine is hot and at idle in gear(900rpm). How much volume is getting to the bearings at 5psi, I don't know... Initial start up the gauge goes to 75.... after I get heat in the engine (160-180) and back out of the shop and it will idle on its own at 900 rpm the oil pressure is still reading in the 40-50 psi range. Seems normal enough there. The drop is only after I run the car and get the water up to 190-200 (who knows where my oil temps are at this point).
If a galley plug was missing, or the pump was sucking air or the clearances are 2X the size I measured.... Would it still be able to get 75psi at WOT and hold steady and not fluctuate? You would think you would see some sort of fluctuation.
I have 3 different gauges to try. I'm going to hook them all directly to the oil filter housing. Spin the pump with a drill while the engine is cold and see what each of them read. RPM will be a constant. Oil temp will be a constant. The only variable will be the gauges.
This engine used to have 10 psi hot at idle when my mains were @ .0025".. The only thing different during the rebuild is using cleveland mains to open up the clearance to .003"... The engine DID take out the cam bearings but I do not believe that was a oil related issue due to the fact that there were no signs of oil starvation to the cam bearings, the main bearings looked as good as the day I put them in and the rod bearings also looked beautiful. You would think if I had a oil delivery issue the rod bearings would be the first to go.. Especially when I was spinning the engine to 7500 rpm..
Pressure is indeed the measurement of resistance, so you are correct that if it had a big dump, you should not have cold pressure, or any pressure later
However, think about how an FE measures oil pressure, it is essentially (except for a short run) right after the pump and just before all the galleys that go everywhere
Based on that, there is always going to be some resistance to flow from the galleys and intersections themselves, to include the filter mount itself as it exits a round about pattern.
5 psi is low hot, real low, silly low...so I do understand that you are trying to work through this logically, but it takes something real loose, wrong oil, real loose oil pump....when it's cold, that resistance enroute to the moving parts is greater, when the oil thins it's less, but it isn't right
I'd be sure a second gauge says the same thing, I'd likely swap out the oil and filter to something different, but if it didn't change, I'd be looking inside.
I do like your plan, it also allows you to listen through the distributor hole, a plug leaking will usually gurgle