Cessna 400 pilot reported noticing the oil pressure indication drop down to 20 PSI before deciding to divert. The pilot was on the wrong frequency for the diversion airport and not cleared to land before touching down.

Narrative

I was cruising 7500 ft above Location A; enroute to ZZZ1 in Location B when I noticed my oil pressure had dropped from 55 PSI down to 30 PSI. I was concerned about engine performance and felt I needed to land at the closest airport; which was ZZZ; 15 miles behind me and to the west. I turned and headed toward ZZZ; I pulled back power and started a long descent. I intended to contact ZZZ Tower to inform them of my situation. I was very focused on my flying and oil pressure continued to fall to 20 PSI. At this point I felt I was in a situation and each time I looked for the Control Tower number my attention focused instead on ATIS and of course I got no response. As I approached ZZZ; I was focused at the north end of the runway; the closest and most direct route to the ground; I repeatedly tried to notify ZZZ Tower but was using the wrong frequency and got no response. I continued to fly the plane and needed to make only a small left-hand turn which was the least amount of maneuvering necessary to land.