The reason I installed a mechanical oil pressure gauge is because the dashboard ones are slow and inaccurate at best.
I was gonna buy a real tester but a dashboard gauge was less money. I 'T'd' the line so that my stock gauge still works. The Dash gauge is about 45 seconds behind the mechanical which responds nearly instantly to increasing RPM
When cold, the max PSI is 68 PSI at 800 rpm, or at 4000rpm. 68psi is the oil pump relief pressure.
When hot this drops to ~63 psi
But at 550 rpm hot as it gets idle, I get 18 PSI with M1 TDT 5w-40 in the sump.
63 PSI is reached, when hot, at 1900 to 2100 rpm.
My factory service manual says 8 to 40 psi at idle is acceptable.
Some oils shear out of grade with age and abuse.
Sometimes fuel dilution in the oil will thin the oil and reduce it's viscosity so less psi makes it to the sender.