1996 Mazda Millennia uses 2 quarts of oil between oil changes

1996 Mazda Millennia V-6 mileage: 75,000. I had the 60,000 mile service performed by independent shop specializing in Japanese cars. I have had oil changed every four to five thousand miles. Valve cover gasket replaced six months ago. The car seems to use about two quarts of oil between oil changes. I had the car check by the Mazda dealer and they found no leaks and no apparent reason for oil usage. I have not noticed any blue smoke or smell from car. The dealer offers no advice. Have you any idea what is happening to the oil and should I be worried about engine damage?