Pilot reported during cruise the engine began running rough and the oil pressure rapidly dropped. Pilot diverted and performed an off-airport landing.

Narrative

On Day 0; I was aboard a single-engine high wing aircraft with another pilot when we experienced an engine issue that led to a precautionary off-airport landing.During a fuel stop at ZZZ; we noticed oil on the front wheel fairing. This was the second occurrence of this issue during my time flying this aircraft. Upon inspection; the engine oil level measured 5.5 quarts; down from the 6 quarts we had at departure from ZZZ1. I contacted the aircraft owner via video call to report the situation. As in a previous incident when another pilot and I encountered a similar issue while ferrying the aircraft across the country; the aircraft owner was unsure of the cause and advised us to monitor the engine in flight.To further assess the issue; the other pilot and I performed an extended run-up at ZZZ; running the engine at 2200 RPM for over 15 minutes while monitoring oil pressure (75 psi) and temperature (179°F). After shutting down; we observed no new oil leaks. We waited an additional hour before rechecking the oil level; which remained at 5.5 quarts. We then added one quart to bring it to 6.5 quarts and inspected for any further leaks but found none.The other pilot was PIC while I was the safety pilot. We circled and climbed to 8;500 feet above the airport; continuously monitoring engine gauges for about ½ hour. Oil pressure remained between 74-78 psi; and temperature stayed between 175-179°F. Observing stable conditions; we proceeded to ZZZ2. Throughout the flight; engine performance remained normal. We continued to ZZZ3; then ZZZ4; before landing at ZZZ5 after approximately 2.5 hours.At ZZZ5; we refueled and conducted another thorough inspection. The oil level remained at 6.5 quarts; and no new oil was found on the front wheel fairing. As a precaution; we added another quart of oil before taking off for the next leg.I was PIC for this portion of the flight; with the other pilot acting as safety pilot. About two hours in; while cruising at 6;500 feet; the engine began running rough; and oil pressure rapidly dropped from 75 psi to 10 psi. However; oil temperature remained stable at 179°F. We immediately turned toward the nearest airport; enriched the mixture; ensured the fuel selector was in the 'both' position; and followed emergency procedures while contacting ZZZ Approach for assistance.Due to night conditions; we were unable to visually identify the dirt airstrip ATC was directing us to. Although engine power remained at 2200 RPM; we were unable to maintain altitude. After approximately 15 minutes; we committed to an off-airport landing in a dirt farm field.During rollout; the right wing clipped the top of a small bush; causing a dent and breaking off the plastic end piece of the horizontal stabilator. The engine was still running when the aircraft came to a complete stop.