Have you noticed physical evidence of an oil leak, such as seepage stains on the engine, or on the ground underneath the car? Give the car a good run, park it, place a large sheet of cardboard or newspaper underneath the engine/transmission area, and allow the car to sit for several hours (be sure to weigh the cardboard /paper down at the corners). Obviously, the presence of fresh oil on the paper would confirm the existence of a leak, and the location of the spot relative to the engine would be helpful in locating the culprit. Any evidence of oil burning, such as blue smoke on startup or on heavy acceleration?