Good oil pressure when cold and lousy pressure when warm is indicative of a fatigued oil pressure regulating valve spring. Almost no one alive knows this. People rebuild engines when they do not need to do so - and find afterwards that the problem persists - because of a spring that has lost it's temper.
Running in hot oil year after year the springs fatigue and need to be replaced. I just did it for the second time - well it did last me 14 years this time but it was pooched all right - put the new spring in and bingo all is good again.
This fix was common to pre second world war cars. There were spring kits available - just try and find one today. Not easy to do.
Oh - in case you are wondering we have had our engine since 1979 and it runs like new - it was built in 1974. Almost forty years old.