Why does a car burn oil?
During normal engine operation, all engines use oil to lubricate all of the internal moving parts of the engine like the bearings, camshaft, crankshaft, timing chain, valves, lifters, piston rods, and piston rings. There are two different ways that the engine will burn oil.
First, the engine uses oil to lubricate the valves so they can move up and down inside the cylinder heads freely without binding up. The oil gets on the valve stem and as the valve is opened, the valve guide oil seal wipes off most of the oil, but not all of it. As the valve continues to open the oil left over after the seal took off, the excess is used to lubricate the valve guide. Then as the valve is opened all the way, any oil left over will get pulled into the valve passageway and go into the combustion chamber for the intake valve and into the exhaust for the exhaust valves. The small amounts of oil loss through the valves is considered normal during typical operation of the engine and will be about a quarter of a quart of oil or less for every 1000 miles. Any more oil loss than this is not normal in most engines.
Secondly, the engine uses oil to lubricate the piston as it moves up and down in the cylinders. The piston in most engines will have three rings on it. The top two rings are compression rings. The third ring is called an oil control ring. Each ring is in its own groove around the piston. The compression rings are used to hold most of the compression in cylinder hence the name compression rings. The oil control ring controls the amount of oil that gets up to the two compression rings. As the piston moves up the cylinder, the walls are sprayed with oil to lubricate the walls and piston. As the piston moves down, the oil control ring will scrape off most of the oil on the walls but leaves a little behind to lubricate the two top compression rings so they do not bind or score the cylinder walls when the piston moves up in the cylinder. This normal lubrication process will allow a small amount of oil to get past all three rings and into the combustion chamber, then out the exhaust through the exhaust valve. The normal amount for each piston to use is less than an eighth of a quart of oil per 1000 miles. This will equal to less than a quart of oil to be used for an eight cylinder engine. Any more oil loss than this is not normal by most manufacturers standards.
Therefore, during normal engine running conditions an engine will burn about a quart of oil or less for every 1000 miles. Your engine may burn a little more oil if it runs in severe operating conditions, but it may also burn less oil if you use it on the highway cruising all the time. I have seen some engines use less than a quarter of a quart between oil changes and others use a quart every 1000 miles. The manufacturer will say this is normal for either one.
Most excess oil usage is due to the oil control rings; if excessive oil gets by these rings, then it will go out the tailpipe. If the positive crankcase ventilation system (PCV) is clogged or not operating correctly, it will cause excessive pressure to build up in the crankcase, causing oil to be consumed at a faster rate.