A Learjet 60 flight crew shutdown the left engine shortly after takeoff due to low oil pressure and quantity. The oil fill cap had not been properly secured.

Narrative

After deice; we taxied to Runway 36 and were cleared for takeoff. I applied full power and accelerated through V1 and rotated at Vr. During rotation; the left 'low oil pressure' light illuminated. At positive rate; I commanded gear up. I verified with the gauge that the left engine oil pressure was indeed low and pulled the left engine to idle. At 1500 FT I verified that the left engine was indeed the culprit; double checked the good status of the right engine; and shut down the left engine. I treated the emergency as an engine failure (even though the engine had not failed) and called for the engine failure on takeoff after V1 checklist and declared an emergency with the tower. I turned on the autopilot and announced to the passenger that we had intentionally shut an engine down due to cockpit indications; and that we would be returning to land. The SIC ran the engine failure checklist; followed by the engine securing checklist. I asked for a delay vector so we could run our checklists. As the SIC finished the securing checklist; I asked for vectors to the ILS Runway 36. I was notified that the ILS was OTS and was offered vectors for the RNAV Runway 36. We discussed our overweight landing; and opted to land as soon as we could. Burning fuel would have taken almost an hour. The SIC completed the descent; approach; and single engine landing checklist. We executed the approach and landed softly on Runway 36. We taxied to our FBO and shut down. After shutdown; I exited the aircraft and noticed the tail covered in engine oil. I immediately notified our office; the chief pilot; director of operations; and director of maintenance. While I was on the phone; my SIC and the mechanics inspected the engine and found that the oil dipstick was not secured. The mechanic admitted to the SIC that he had checked the oil level. At that time; my SIC told me that he had not checked the oil level during his preflight inspection. Normal company procedure is to check the engine oil 10 minutes after shutting down the engine. I normally check the engine oil if I know that maintenance has serviced the engine. At this time; we have no procedures in place to notify the pilots of routine maintenance action performed on the aircraft.