You just need to find an older Repair shop that still has the old test gauges. Every so often the Onboard computers are replaced with more capable models. The connectors are different, so you need the proper old style diagnostic scanner.
You can still use the old style Vacuum gauge to diagnose some things and set the carburetor. You should have an electric/vacuum solenoid on the carburetor to compensate for the A/C running at idle. Someone may have disconnected it, or adjusted the idle speed missing the solenoid.
You do not want to have high idle rpm just because you have an A/C; its suppose to adjust when the A/C cycles off and on and maintain a stable idle.
That said, the oil pressure may be more steady at idle if the RPM is steady. Now for fluctuating oil pressure at high RPM, you may have a bad fit inside the oil pan with the oil pump pick-up tube. There are pressure relief springs in some oil pumps, oil filters have relief springs, and your oil pressure gauge may be wrong.
Oil pressure gauges are not a major expense, or even have a Mechanic hook up a test gauge temporarily to verify pressure.