What you describe is exactly how a REAL oil pressure gauge functions. Oil pressure will be higher in the engine when it is first started cold, then decrease as the oil warms up. When the engine is idling, oil pressure will be low. When the engine RPMs increase, the pressure will go up to "normal" operating level, probably somewhere around 40-60psi. This is why big diesel engines have a "high idle" setting ...they have very little oil pressure at normal idle speed so when idling for longer periods then engine RPM should be kicked up to 1k or so where oil pressure will be "normal." You should only be concerned about your oil pressure if the reading at normal operating RPM is not in the normal range ...oil pressure IS LESS when the engine is idling.
Very few vehicles have real oil pressure gauges now. When I questioned that several years ago on a Ford vehicle I owned, I was told that mfg'ers got tired of wasting money on warrantly claims/warranty service about low oil pressure readings that were caused by nothing more than customer ignorance of how engines operate. Many vehicles today have an "idiot light" ...so called because it comes only AFTER the engine has been trashed due to low oil pressure rather than warning the operator of a possible problem by displaying gradually falling pressure over time. Some vehicles do have a gauge type oil pressure indicatior in the cockpit, but most of them in reality have only the "idiot light" type sensor ...it either reads pressure or it doesn't, and the needle on the gauge is either at normal level or it is not.
My Cummins ISC came with a real oil pressure sender and gauge. But Cummins had many problems with the variable sending units. When mine failed under warranty in 2004, causing multiple flashing lights, chimes and "shut engine down immediately" warnings (as did many others), Cummins replaced my variable sensor with the idiot sensor.