Part 91 pilot reported during cruise the engine oil pressure dropping. They contacted ATC and landed as a precaution.

Narrative

The pilot/owner and a non-pilot-rated passenger were on an IFR cross country flight from ZZZ to an intended destination of ZZZ2 with a filed altitude of 8;000 feet MSL. The route was a mixture of low-altitude airways and VOR-to-VOR navigation. After the preceding flight the day before; the pilot had noticed an unusual spot of oil blow-by on the left nose gear door; near the oil breather tube. The aircraft had also consumed approximately 1 quart in 3 flight hours during the preceding flight; which is an unusually excessive oil burn for that aircraft. The pilot added a quart of oil immediately prior to the incident flight. The pilot departed ZZZ1 with approximately 6.5 quarts of oil in an 8 quart maximum system. The Lycoming IO-360-A1A as installed on the Mooney M20P series is known to have an oil breather tube set relatively low; so any oil more than 7 quarts is known to blow out of the system. Therefore; the pilot was accustomed to flying the aircraft with between 6 and 7 quarts of oil. The flight from ZZZ1 to ZZZ2 was largely uneventful. At approximately 2 hours 45 minutes elapsed time from engine start; the pilot noticed an unusually low oil pressure reading of 50 psi. The normal oil pressure reading in that aircraft at cruise power is just under 75 psi. The pilot continued to monitor the engine instruments closely over the next few minutes. All other instruments read normally. Shortly after the pilot noticed the relatively low oil pressure; the oil pressure momentarily dropped by approximately 25 psi with a corresponding increase in engine roughness. After a few seconds; the oil pressure increased back to approximately 50 psi and the engine smoothed out. The engine repeated this cycle several more times approximately every 3-5 minutes for the remainder of the incident prior to the precautionary descent described later; with each cycle yielding slightly worsening oil pressure.The aircraft was approximately 3 miles east-southeast of ZZZ when the low oil pressure/roughness cycle occurred again; but with the oil pressure needle momentarily touching the oil pressure minimum idling limit line of 25 psi before recovering to a higher reading. The pilot decided to divert to ZZZ for a precautionary landing; although the engine was continuing to run generally well. Based on his observation of the oil blow-by on the left nose gear door the previous day; the pilot correctly surmised that the low oil pressure reading was indicative of low oil quantity. The pilot notified ATC that he had minor engine trouble and advised of his intentions. ATC cleared the pilot to divert to ZZZ and to descend at pilot's discretion. The pilot continued to operate under IFR until he descended below a broken cloud layer at approximately 5;000 feet and acquired ZZZ visually. The pilot executed a controlled spiraling descent to ensure that he remained within gliding distance of ZZZ in case the engine trouble resulted in a loss of power. The pilot did not declare an emergency; as he felt that the situation did not require priority handling or emergency ground equipment. After VFR cloud clearance/visibility requirements were met and continued VMC was assured; the pilot canceled his IFR flight plan. ATC cleared the pilot to switch to CTAF and squawk VFR; and further requested that the pilot notify the TRACON via ZZZ clearance delivery frequency after he landed safely. The pilot noted that the oil pressure recovered to approximately its normal reading of just under 75 psi during the precautionary descent; and no further episodes of engine roughness occurred. The pilot was able to land uneventfully at ZZZ; taxied normally; notified the TRACON controller of his safe landing; and shut down the aircraft without further incident.After allowing the engine to cool; the pilot checked the oil quantity. Only 2 quarts of oil remained in the engine. Additionally; the spot of oil blow-by on the left nose gear door had returned. Furthermore; there was wispy smoke coming from the oil dipstick opening when the pilot opened the dipstick and the oil itself appeared black and sooty. The oil had been changed within the last 15 flight hours; and an oil sample had been sent for analysis at that time with no apparent issues. An A&P IA investigated the aircraft in the following days; and discovered that three of the four cylinders had signs of serious thermal stress from overheating. The #3 cylinder had failed completely with blown rings; and two other cylinders were close to failure. The engine only had 180 hours on it since major overhaul. The A&P IA and the engine repair shop initially believed that the pilot/owners had been continually operating the engine with an excessively lean mixture and/or excessively high power settings; but that belief was inconsistent with the aircraft's known average fuel burn of 10.5-11 gallons per hour throughout the engine's post-overhaul history combined with the pilot/owners strict adherence to 75% or less cruise power settings. Three CFIs; including one CFI specializing in type-specific Mooney training; who had flown with both pilot/owners corroborated the pilot/owners' statements that the engine had consistently been operated correctly with an appropriate; conservative rich-of-peak fuel mixture; at power settings at or below 75% of the engine's rated power. However; in addition to the required engine instruments the aircraft was only equipped with an analog cylinder head temperature gauge and analog exhaust gas temperature gauge; both with questionable accuracy. Additionally; the aircraft had routinely been operated between 12;000-15;000 feet MSL despite being naturally aspirated. The aircraft was also equipped with standard; non-pressurized magnetos.After extensive discussion between the engine repair shop; the investigating A&P IA; the pilot/owners; and a third party aircraft piston engine specialist; the overheating and subsequent thermal damage to the engine is believed to have been caused by ongoing magneto timing problems caused by ozone contamination in the magnetos from frequent operation above 12;000 feet MSL. The pilot/owners were not aware of the possibility of ozone contamination causing non-pressurized standard magnetos to have timing problems. Contributing to this was the fact that the aircraft was not equipped with a digital engine monitor; which resulted in the resulting overheating problem never coming to the pilot/owners' attention due to the questionable accuracy of the analog engine instruments. The oil pressure surges with corresponding roughness is believed to have been caused by the low oil quantity; which was secondary to the thermal damage related cylinder failures. The pilot's decision to divert to ZZZ based on the oil pressure irregularities; despite being within 45 minutes of his intended destination of ZZZ2; was a sound decision. In any likely IFR descent profile from the area of ZZZ to ZZZ2; there would have been a large envelope in which the aircraft would not have been within gliding distance of any airport. Had a total loss of power occurred at a relatively low altitude between ZZZ1 and ZZZ2; the pilot would have been forced to make an off-airport landing which could have easily resulted in a negative outcome for him and his passenger. The TRACON controller at ZZZ3 assisted by accommodating the pilot's requests; even though an emergency had not been declared.The incident was mitigated by the pilot's routine monitoring of the engine instruments. The incident probably could have been avoided entirely if the pilot/owners had been trained or educated that regularly flying without pressurized magnetos or electronic ignition above 12;000 feet MSL can cause ignition timing problems and subsequent overheating.