M-20 pilot reported loss of engine oil pressure and diversion to nearby airport.

Narrative

VFR flight in VMC returning from ZZZ1 to ZZZ (home airport); operated Part 91 for personal GA purposes. Pre-flight inspection was performed and normal. Engine condition visually inspected and no abnormalities. Oil was visually checked and topped up to 6.5 qts plus Camguard additive. (Oil consumption over last 12+ months has been consistent; monitored closely; with 25-30 hour oil changes more frequent than manufacturer recommendation; and with routine laboratory oil analysis). No significant abnormalities on the outbound flight to ZZZ1. On the return flight from ZZZ1 to ZZZ; aircraft performed normally. Only observed condition was sluggishness on take-off from ZZZ1; but attributed to density altitude (DA over 8;000 ft). Climb to cruise altitude 8;500 was normal. Oil; EGT; and CHT temps all normal; within 'green' arcs. Aircraft was leaned to 150F rich of peak using 4 cylinder engine monitor.Approximately 1h20m into flight; a descent was commenced from 8;500 in anticipated of visual arrival to ZZZ. ATC services were VFR flight following with ZZZ TRACON. Passing through approx 6;000; the oil pressure gauge was observed to show slight 'flickering' movement; within the green (normal) arc/band. Very quickly; the flickering needle dipped into the yellow range. Myself as PIC notified ATC that we had a possible oil pressure irregularity and would be diverting to ZZZ2 for precautionary purposes. ZZZ2 was the closest airport; approx 10 miles west of present position. As a towered field with known maintenance facilities on airport; it was deemed the most appropriate diversion until the oil pressure indication could be verified. Shortly after initiating the diversion; the oil pressure gauge dropped to the red line. The engine was still running and exhibiting normal behavior (EGT; CHT; observed noise). Out of abundance of caution; ATC was further notified of the redline oil pressure and priority handling was requested in the event of an engine failure while still in flight. Initially set up for ZZZ2 Runway XX however due to excess altitude; changed to Runway XY.Landed without incident and taxied off the runway under own power. Was met by airport fire rescue services. A visual inspection of the aircraft showed some oil streaking on the right side of the engine cowl (exterior); substantial oil leakage inside the cowl on the right side; and oil visible on the left & right gear door and belly. Arrangements were made to tie the aircraft down at ZZZ2 and for local A&P to investigate the cause of the visible oil.