Why would your car use up lots of oil?

Well, it's going somewhere. Put a piece of cardboard under the car and let it idle for 10 min. Then shut it off and let it sit overnight. You got spots on the cardboard, you're leaking it. Old engines have exhaust gas escaping by the pistons and into the crankcase (called blow-by). Once it gets to the point the PCV valve can't handle it, it pushes out of the various seals on the engine and makes leaks all over. Only way to fix that is rebuild the engine. On the other hand if you don't have pressure coming out of the crankcase, you likely just got a bad seal. The crankcase side of the PCV valve is a good place to check for this. How much is too much? um... if you can feel 'air' out more than it's going in, it's too much. If not, you're burning it. If people honk when they pass you from the cloud of smoke you are leaving, you are buring oil. Possible causes are a badly worn engine with oil rings worn all the way down, or bad valve seals leaking into the exhaust or intake. Common failure point for high mileage cars is the rear main seal. It sounds like a big deal because it is. It's located where the spinning shaft leaves then engine and goes into the transmission and is the largest turning seal in the engine. Replacement requires removing the transmission. Your engine will leak between the engine and the transmission for this one. Enough.