Cirrus SF50 pilot reported an engine oil pressure warning message during cruise flight; then diverted to an alternate airport and landed. The pilot discovered the oil cap was not in place after performing oil servicing at the departure airport.

Narrative

I had planned to fly from ZZZ1 to ZZZ2; stopping at ZZZ2 for the night with a fuel stop at ZZZ3. About one hour into the second leg from ZZZ3 to ZZZ2; I had a red Engine Oil Pressure CAS (Crew Alerting System). As per the checklist I reduced to idle thrust. This did not improve the oil pressure; so I requested priority handling and prepared to divert. The oil pressure dropped to around 24 PSI and stabilized there (Person A later informed me they were seeing 18 PSI). The engine continued to produce thrust all the through landing and taxi off the runway.I originally planned to divert to ZZZ4 straight ahead. This would make maintenance convenient. I had a minute or two of confusion when the avionics showed my ETA to ZZZ4 at ~22 minutes vs my original estimate of ~10 minutes and first thought I had put something wrong in the flight plan. Then I realized it actually is 22 minutes at 120 KIAS glide with a headwind. Shortly after; I decided to go to ZZZ instead because it was 10 minutes closer and it became apparent ZZZ4 was outside of glide range. I considered CAPS (Cirrus Airframe Protection System) deployment over a lake to be unsurvivable; but a CAPS deployment on the way to ZZZ over land in VFR conditions would allow SAR to reach me before hypothermia (airports in the area were reporting between -10F and 0F).I rejected the traffic controller's suggestion of ZZZ5 due to IMC conditions and possibility of icing.I flew a non-standard approach into ZZZ RNAV XX; electing to stay way high until about 5 mile final to preserve glide distance. Inside of that I flew a normal approach; which uses ~30% engine thrust. I neglected to run the approach checklist; so I didn't notice my landing light was off until in the flare. This was my first time landing at night with the landing light off. The runway edge lights were functional.The ultimate cause was obvious after shutdown. I observed the oil level was empty and filler cap was not in place. It is likely the cap was NOT replaced after the oil top off done earlier that day.I had started the preflight inspection at ZZZ1 a few hours ago inside the hangar because the FBO personnel had not towed the plane out yet. But they showed up a few minutes later; which cut my preflight short. I quickly checked the oil level (as I have done on every first leg) but did NOT make any attempt to check the cap (in the past I have skipped checking for various reasons; including lack of light or a step stool high enough to get a clear look). I did notice the oil level seemed to be well above where I had left it the previous flight (close to the add line); but well below the full line. This didn't make that much sense; but I estimated the oil would last around 10 flight hours which was sufficient for the day; and didn't think any further of it.