For whatever it's worth:
When I first got my BN2 with around 85,000 miles on the lower end, the oil pressure ran around 55 psi cold, 25 hot idle, & 45 hot run at 3,000 rpm.
I experimented with shimming the presure relief plunger spring .160". The pressures were the same, no change. Next I
added a .360" long spacer in place of the .160" spacer. The hot oil pressures went up to 40 at idle & 60 hot run at 3,000. I expected the hot run pressure to go up because of the additional spring pressure raising the pressure relief setting. What I didn't expect was the increase in idle pressure since 40 is well below the relief blow off setting of 60. Pressure now never goes below 40 at idle, or above 60
running. Of course idle pressure drops from 60 to 40 as the oil gets hot but It used to drop to 25.
I came to the conclusion that the regulator valve was leaking (bypassing) all of the time (not fully seating) & causing the low idle pressure. The additional spring pressure caused the regulator valve to fully seat & raised the idle pressure which was previously leaking through the non seating valve. The first try shim of .160" apparently
was not enough additional spring pressure to fully seat the valve.
It is possible that by checking for full valve seating & lapping the valve to it's seat that the low idle pressure would have been considerably higher without the spring change.
On my particular engine, a spring shim of more that .360" would result in the relief valve not uncovering the bypass port due to spring coil bind & complete blocking of the regulator, so .360" was the limit, but fortunately just the right amount.
It might be useful for you to experiment with the regulator valve seating & spring pressure.
Regards,
Dave