I have 2004 nissan Xterra, with 166,000 miles. I am losing about a quart every three days, normal driving. I have already replaced both valve cover gasket. There is a small rear main seal leak but not enough leak to loose 1 quart. There is no big puddle of oil on the driveway. No oil on the back window which would indicate excessive oil leak when the car is moving. No black or white smoke. There is black stuff and water that comes out of the tail pipe at first start in the morning. What is leaking and where is the oil going.