I had a problem with very low oil pressure when the engine was warm ten years ago and it took me a long time to figure it out but I did.
The spring in the regulating valve fatigues over time and looses it's force - despite any adjustment to the valve. In old cars they has spare spring kits available for adjustable regulating valves but almost no one knows about these anymore.
I put in a new spring (from an assortment I was given) and cut it to an appropriate length. I snugged it a bit after first contact and that was the end of my warm low oil pressure problems.
I had tried everything including a light rebuild, thicker oil etc. but all that was wrong was a simple worn out spring.