BE35 pilot reported loss of oil pressure in flight and after landing and parking; oil was observed along the entire fuselage. Inspection revealed an STC-approved quick drain oil plug was missing.

Narrative

The aircraft at some point in the past had a quick drain oil plug installed accessible via the nose gear well. This plug was safety wired to an angled piece of aluminum that was riveted to the drain plug cover plate that is secured to the airframe. These aftermarket STCed quick drains are made of aluminum. The aircraft recently came out of annual and had about 2.5 hours in service at the time of the event. During the annual all engine mounts were replaced. The oil was changed. And a visual inspection of the drain plug safety wire confirmed it was attached to the aluminum bracket in the nose gear portion of the airframe.Upon entering the pattern at ZZZ on a return flight from ZZZ1; it was observed that the engine oil pressure was at the lower limit at 30 PSI. After landing and while taxiing to the FBO it was observed that oil pressure dropped to 0 PSI. Engine temperature was normal and no anomalies were detected. Upon shutdown and exiting the aircraft engine oil was observed; of oil covering the entire undercarriage of the airframe from tip to tail. Oil dropped; puddled; and pooled under the aircraft. Upon further inspection it was discovered that the oil plug was missing. The hole in the aluminum bracket safety wire attachment point was worn through; negating the usefulness of the safety wire. The service shop at ZZZ replaced the plug with an original certificated plug and safety wired to the engine and NOT the airframe.It's believed that the failure of the drain plug to stay safely secured was caused by it being secured to the airframe; which with the engine running was able to vibrate its way loose. Further investigation revealed that it's a common practice to secure this aftermarket quick drain plug valve to the airframe because it extends through the access plate; making it convenient but not effective. In hindsight I'm glad we didn't drop the oil while climbing out over ZZZ2 in Day 0. Otherwise I might not have been able to write this. I hope this information is helpful and gets disseminated as soon as possible. I shared my experience with the service personnel that performed my annual. They had an A36 Bonanza in their shop also with the same drain plug secured to the airframe. Upon hearing about my experience they immediately removed the safety wire from the airframe and secured it to the engine.