Vehicle engines require motor oil to lubricate and protect internal parts from wear. When the piston moves down its cylinder wall, a thin film of oil is left on the cylinder wall. Part of this film is consumed in the combustion process. As a result, varying rates of oil consumption is considered normal in all engines. Leaks from gaskets and seals also contribute to loss of engine oil.
NOTE: Most vehicle manufactures recommend checking the engine oil every time you fill your fuel tank.