Because the Piezo-Electric (Non-Analog) versions are capable of measuring quite discreetly... they will show *Much Finer Grain Details* whereas the Oil in the Analog Fluid Line Versions might jump up an down in levels a bit until the Engine RPM Levels Off....
mostly because of Residual Air left inside of an un-bled OPS Nylon (or Copper-Coil) Line.
But if you actually want to know whether the Main Bearings are Bleeding Out Oil from excessive wear or if the Gerotor Oil Pump "Viton" "O" Ring has Failed and is causing a Loss of Vacuum in all RPM Ranges... the True Mechanical Analog Oil Pressure Gauge always provides this Proof, one way or another...
just as soon as the Crankshaft begins Turning.
The very instant those Gerotor Oil Pump Gears begin to move... the Oil Volume is directly driven right through the system. In terms of Pure Hydraulics...
if you push on the Oil contained within any Oil Gallery...One Inch... the Oil present everywhere inside the Oil Column will also move ...One Inch... Instantaneously... because all Liquids are In-compressible: