Classic error...the gorilla who installed this oil drain plug extruded the aluminum sealing ring, the result being a slow leak. This is a car, but the same principle applies to various seal rings on your Lycoming.
Nasty. I'd bet the same shop torques the wheel lug nuts to 300 ft-lb with an impact gun when the spec calls for 95. I won't let a dealer, or even an independent mechanic near my car.
On Ex-Wife's car, I found the drain plug CROSS THREADED the first time I changed the oil. Fortunately Toyota was smart enough to make the thread in the pan harder than the plug. New plug worked fine without needing to re-tap or replace the hole in the pan.