All cars burn oil. It is just the rate of burning that differs. When your vehicle changes from burning a litre of oil in thousands of kilometres to one that burns a litre in hundreds of kilometres, you have a problem.
Engine oil is the lifeblood of the internal combustion engine. Not only does it prevent the wearing of moving parts but it also keeps those parts from getting too hot as well. Oil is a coolant as well as a lubricant.
If you own a newer vehicle that has an oil life monitor remember that the oil life monitor may let your vehicle go a lot longer between oil changes. In doing so you must remember that you very likely will have to add oil between changes. Many vehicles can go 5000 kilometres and only burn a litre of oil but not many can go 10 000. You have to check your oil level. There are very few vehicles that monitor oil level and running your vehicle low on oil can be a disaster.
Oil is burnt in the combustion chamber along with the fuel. The oil is used to lubricate the piston moving up and down in the cylinder to prevent metal to metal wear. Each pass of the piston leaves a microscopic layer of oil. This exceptionally thin layer of oil is burnt up in the explosion of the air and fuel mixture.
The amount of oil that ends up left in the combustion chamber is controlled by the piston rings. These rings are used to seal the piston in the cylinder to maximize the squish of fuel and air while distributing only that microscopic layer of oil. When the rings no longer do their job well then extra oil gets left on the cylinder walls.
When your rings are worn out repairing that oil burning problem will be expensive. There are other oil burning scenarios that are simpler to repair. Some of them may be just plain poor maintenance practises.
Never overfill your engine oil. This will almost always result in extra oil being burnt. It can also cause extra wear in your engine. When the oil level is too high the crankshaft will froth up the oil and fill it with air bubbles. Frothy oil is a poor lubricant. It also splashes extra oil on the cylinder walls and leaves too much oil for the piston rings to scrape off the cylinder walls thus leaving more oil to burn off.
A somewhat sudden change in oil usage that is not an external leak can indicate there is a problem in your crankcase ventilation system. The crankcase ventilation system prevents pressure build up inside the engine case. Each explosion that forces a piston down has a little bit of explosive pressure that sneaks by the piston into the engine case. These gases that sneak by the piston must be allowed to go back into the engine to be burned again. The plumbing system that meters this flow can fail.
If crankcase ventilation flow is too high oil will be pulled from the crankcase into the engine. If crankcase ventilation flow is too low pressure will build in the crankcase and force oil out around seals or even push seals out and cause external oil leaks.
Diagnosing a faulty crankcase ventilation system is not typically very difficult but repairing the system can require some dis-assembly to access components that are hard to get to.
Infrequent oil changes and use of low quality or the wrong engine oil will also result in high engine oil usage. Worn out engine oil will allow deposits to form in the engine. Sometimes piston rings get deposits on them that prevent them from controlling the oil film.
Suffice to say, watch your engine oil usage. Any change is an indication that trouble is brewing. Being proactive can be the difference between replacing a simple positive crankcase ventilation valve or completely overhauling your engine.