A car burns oil for several reasons. The problem usually occurs with older cars that have a lot of miles on them, when the engine has endured much wear and tear. There may be a leak around the valve covers, for example. Newer cars burn very little, if any oil at all, between oil changes every 3,000 miles. But even newer cars can burn oil if their engines haven't been maintained properly.
Inspect the valve cover(s)--there may be one or two--on the engine for oil leaks. If there's oil leaking along the valve covers where they meet the engine head, that's probably why the car is burning oil. Because this area is not under pressure, the leak will be rather slow.
Check the tightness of the valve cover bolts. If the bolts are loose, that could cause a car to burn oil. Use a torque wrench to tighten them. Check the car's repair manual to find out how tight the bolts should be. Be sure the bolts aren't too tight or the valve cover could warp or crack.
Remove the valve cover by loosening the bolts. Pull the gasket out of the groove in the valve cover. If the gasket is deteriorated, you may have to pry it off with a flat-blade screwdriver. Also, remove the inner o-rings with your fingers. Replace the valve cover gasket (see Resources).
Drain the engine oil in the car that's burning oil. Install a new oil filter. Add a heavier-weight oil into the crankcase. Choose a weight that's a grade heavier than the oil you were using. It won't leak as easy because it's thicker.
Add STP Smoke Treatment to the car during every oil change. This product will thicken the oil in the car to reduce oil leaking and burning. Follow the manufacturer's instructions for best results. Basically, you just add a bottle of the STP Smoke Treatment to clean oil in a warm engine.
In a car that has a high amount of miles on it, using one quart of oil every thousand miles can be expected.
A car that's burning oil won't pass an emissions test.