The flight crew of a Twin Navion shut down the left engine and returned to their departure airport due to a low oil pressure indication.

Narrative

Climbing through approximately 25;000 feet we noticed fluctuations in the left engine oil pressure associated with flickering of the master caution. I was the flying pilot and I asked the non-flying pilot to get the abnormal checklist. The oil pressure then steadied in a low pressure position. I asked Air Traffic Control to return to our home airport. I received a direct clearance and a descent. We worked through the checklist; which eventually had us shut down and secure that engine. This was approximately 5 to 10 minutes before touch down. We executed a single engine visual approach to Runway XX. Upon inspection of the engine after landing; it was determined that an 'O' ring on the oil filter cap had split. A new 'O' ring was installed and engine leak checked and was found to be in normal operating parameters. We consulted with the engine manufacturer and were told the engine was okay to operate. Our crew resource management was excellent and we handled the situation with professionalism; as we have been trained to do.