You have to have a leak somewhere! Even if it through the piston rings or valve seals and escaping as smoke, there is a leak! Go under your car (watch out for those cabbies going 60mph!) and look for any visible sign of wetness, especially at the front of the engine and rear of the engine, double check your plug is tight and filter is tight, seem common-but that is the point, it does happen more often than you may think-especially the "double gasketed" oil filter-that will leak like a sieve!