i had a similar leak once, it was a gasket between the oil filter adapter housing (piece that the oil filter screws onto) and the engine block. it started as a small leak, but in two weeks it was losing ~quart every 5 miles. if the oil is leaking down on to your flexpipe (the section of the exhaust that looks like it is braded, and well, flexible) then it is most likely this gasket. If not, check your oil sending unit, which is located directly below the filter adapter housing.
hope this helps