could be a host of things..you can start with a compression test. Higher milage engines will burn oil as it gets past the rings, valve seals will leak...not really something you can remedy, just tolerate as long as that engine lives under the hood. Unless it's a significant amount of smoke and oil consumption I would just look to keep the oil clean (maybe a high mileage oil) and a good filter...keep the general maintenace up and drive the car.