You were right in installing a mechanical gauge to verify the issue. In my opinion, going to a heavier weight oil will just mask the issue. Think about what pressure is; resistance to flow. You could put gear oil in the crankcase and the pressure would be high, due to the resistance of the thick lube to flow. You would see good pressure, but would not have the correct FLOW to all the bearings etc. Just like what you are seeing with the cold engine, except with higher weight oils, they are more resistant to thin out as they get hot so the pressure does not fall off as much - and neither does the flow. That is why cold oil is harder on your engine - resistance to flow.
In my view, 30 psi cold at any rpm higher than idle is too low.
Based on what info you supplied and what you have checked, I think you have one of two issues:
1) Bad Oil Pump (not very likely but can happen).
2) You left a cup plug out of the block oil system and have an internal leak. The first place to look would be the front of the block where there are three 3/8 inch cup plugs one on both sides plugging the lifter oil drillings, and one for the main oil rifle feeding the Mains and Cam Bearings - See the red arrows in attached picture (At the back of the block, there should be three pipe plugs for these drillings).
Also, and here may be your issue, there is a 1/8th inch oil drilling about 5 o'clock from the front Cam Bearing - See Orange circle in attached picture. This is a cross drilling to oil the Distributor Gear and where it pilots in the block.
On the earlier 289s this was plugged with a cup plug in the front. On later small blocks, the drilling was left open but was covered by a different design Thrust Bearing. If you have the earlier design Thrust bearing, and no cup plug, there would be a pretty good internal leak. Symptoms would be as you described.
Bad part would be you would have to remove the front timing cover to check....
Good Luck.
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