Mooney M20 pilot reported a loss of oil pressure during cruise and the need to divert to a nearby airport for landing. Upon inspection; it was discovered a missing rocker cover caused an oil leak and only a small amount of oil remained in the engine.

Narrative

I was VFR on-top on an IFR flight plan from ZZZ direct to ZZZ1. My airplane had two cylinders replaced at an aircraft maintenance shop about 5 tachometer hours previously. The engine oil level was measured normal (7 quarts) before the flight. At about the ZZZ state line; I noticed fluctuations of the engine oil pressure indication. This was concerning; so I made a 180 turn and informed ATC that I was returning to my departure airport. I then observed the oil pressure gradually decrease toward the red line. I determined that ZZZ2 was the closest airport; and informed ATC that I was diverting there. Fortunately my engine kept running without high temperatures and I landed at ZZZ2 safely. Exiting the airplane; I saw a lot of oil behind the left side of the cowling. After calling flight service to inform ATC of my landing; I contacted a local mechanic; and removed the engine cowling. The mechanic found loose rocker-cover screws on the recently-replaced cylinders; one of which was missing the exhaust valve rocker cover; causing an oil leak. Only a small quantity of oil remained in the engine. Obviously; more careful torque-checking of the screws by the mechanics should have been done. I was very lucky to have made the airport before oil starvation and engine seizure; which could have occurred minutes later. In retrospect; I should have immediately diverted to the nearest airport at the first indication of an anomalous oil pressure reading; instead of several minutes later. My current awareness of how this incident could have ended badly will shorten this response time; should i be unfortunate enough to have a similar experience again.