A C310 reported losing oil from the right engine. He shut down the engine and diverted to the nearest suitable airport; where he discovered he had failed to secure the oil fill cap.

Narrative

Shut down engine enroute due to oil discharge. Shortly after reaching cruise altitude of 5500 FT a large amount of oil was observed flowing over the right engine cowling. I immediately diverted to closest airport. While enroute I observed the engine was still operating within normal temperature and oil pressure limits; but I was concerned the engine would eventually stop and/or the oil might ignite on the hot exhaust system so I chose to shut down and feather the right engine. I continued uneventfully and landed. The local A&P checked the engine which still contained 8 quarts of oil. The source of the oil turned out to be a loose oil fill cap. After adding 4 quarts of oil; we test ran the engine; it was developing full power and operating normally. Since the shut down occurred before any abnormal temperatures or performance was observed; it was determined that no damage had occurred. I did not declare an emergency; however I had tuned in the nearest ATC frequency just in case I needed to call for assistance. I also tried to raise the local Unicom to let them know I was coming; but I couldn't raise them. When I added a quart of oil to the right engine just before departure; I believe that I forgot to put the cap back on. My standard practice is to lay the fill cap/dip stick on top of the cowling so I don't forget it; however on this airplane the dip stick and oil fill cap are separate. The oil fill cap is on a short chain attached to the engine. When loose; the restraint keeps from losing the cap; but it also keeps it out of sight. Likely this deviation from habit contributed to my failure to properly secure the cap.