there's several reasons it could be doing this ,one is it could be burning it through the heads or the valve stem seals and the other one is its burning it through the rings in the engine,depending on the mileage it has on it it could be something as simple as a set of valve stem seals on it,that will cause one to use about a quart a week if not more,the second most cause of that happening is a bad intake gasket ,that will allow it to pull oil from the engine and burn it also,good luck.