Hap and the Dr. are on the right trail. But let me help put some of this in perspective. The pressure the gauge will reveal to the driver is the pressure in the oil galley along the distributor side of the block prior to reaching all the bearings in the engine. The pressure on the gauge is the galley pressure prior to the main, rod and CAM and ROCKER ARM bearings. Don't forget them too. Oil pressure is measured at this convenient location. Clearances and losses all along the oil pressure path determine what the pressure will be at the galley just before what's left goes to the bearings and then drops into the pan to be collected for the cycle to start all over again. The initial and final viscosity of the fluid (oil) affect the losses in the operating temperature range of the engine. Things like STP/Lucas/Motor Honey or whatever the trade name is are pretty much just viscosity improvers, friction modifiers and oil chemistry supplements. They make the oil thicker and slow the clearance loss.
The oil pump itself has clearances and can affect the pressure on the gauge. I have found cracked scrolls on oil pumps that of itself would greatly affect the pressure at the galley.
The face condition of the relief valve plunger can affect its sealing capability and the final oil pressure value once temperature has stabiblized.
Now to bearing clearances. The thrust washers have very little impact on oil pressure as long as the clearance is within tolerance. On an A series engine, this can be up to 0.007" or so for race purposes. However, when approaching this limit, the side clearances of the rods and the clearance between pin boss at the piston and the rod little end need to be checked. Rod side clearance greatly affects oil pressure loss and windage. A good oil pump and low loss piping and filters are a must. Cam bearing, rocker bushes, rod and mains all need to be in specification for the application.
Also, the oil pump needs to be inspected. I have found most oil pumps show up with around 0.006" clearances between scroll and case. And around 0.001" to less between rotor and scroll. I prefer the flat metal back pumps. The MiniSpares performance pumps are good, but I have found a few with not as tight a clearance as I would have expected for selected assemblies. Also make sure the flat sides of the scroll, rotor and pump casings are not scored from debris sucked up by the pump before being filtered. Make sure there are no kinks or flow restrictions in the stock oil path. The oil pump itself is a constant displacement pump. More rpm's, the more displacements and hence you should see more pressure for the same restrictions in the path.
Check your clearances with plastigauge or measurement tools on the rods and mains. Plastigauge is a really good thing. It is a very reliable substitute for using expensive tools. Check your cam bearings for scoring and wear. The center and front bearings will show a ridge in the oil groove area if the bearing is worn. The amount of ridge gives a perspective on wear. Rear bearing does the same, but to a lesser degree. The rockers should be checked for the amount of slop or wiggle with no lash. Also check the journals to make sure you are not on the low side of the range. Same with the rod big ends. Tighten clearances up and I'm sure the gauge reading with improve.
Now, with respect to bearings. I don't have the same adversion to Aluminum alloy bearings as others. Many of the newer Japanese, European and American engines can only use AL based bearings because that is all that is available. These can be raced/used just as reliably. Also, ACL and others have developed some really exotic aluminum alloy overlays to combat the environmental overhead and difficulty using lead or indium materials.
One last suggestion of anyone building an A series engine is to check the alignment of the oil feed hole in the bearing with the drillings/groove in the block. Have seen up to half the hole blocked by the block itself.
Oh, and Cleanliness and Cleanliness. Make sure you clean everything and all the galleys, nooks and crannies where grit and trash can hide.
Resort to additives and high viscosity oils (>50wt) only as a last and temporary strategy. Supplemental ZDDP additives for initial engine operation due to the flat tappet cams is advised, but then should be eliminated and good quality oils with a minimum of 1100-1200 ppm of zinc/phosphorus installed.
HTH
Mike Miller