There's really only one reason for oil to be burning in a car. An internal combustion engine runs very hot, hotter than the burning temperature for motor oil. If oil comes into contact with the hot parts of the engine, that oil will burn. The more useful question is: Why is my oil coming into contact with those hot parts of the engine?
The most common reason for oil to end up on the hot parts of the engine is a leak. These leaks can come from bad joints or seals, warped engine parts or worn valve guides and piston rings. In all cases, oil will leak out of the oil system and into hotter parts of the engine, where it will burn.
If you overfill your oil tank, the excess has to go someplace. That someplace is often other parts of the engine. Just as with a leak, those engine parts run hot enough to burn your oil. You can check your dipstick to determine if this is the problem.
If you have too little oil, it won't lubricate the moving parts of your engine enough. Without proper lubrication, friction in those engine parts increases. Friction creates heat, sometimes enough heat to burn what oil there is. This can create a vicious cycle in which your engine burns hotter and hotter, consuming more and more oil. Use your dipstick and oil gauge to determine if this is the cause of your burning oil.
