Your car's engine is dependent on several liquids to keep it working. It needs coolant (a mixture of water and antifreeze) to make sure it doesn't overheat. It needs brake fluid so pressure can be applied to the brake pads and it needs water for cooling and wind shield washing. While all of these are critical to the car perhaps what the engine needs most of all is oil. If we look at the functions of oil we can understand why it needs to be regularly checked and replace completely periodically.
Oil is sucked up from the oil sump by the oil pump through an intake hose. The oil is then pumped through a filter to remove foreign material that may have collected in the are you oil as a result of tiny fragments braking away in the engine or as leftovers from the combustion process, such as carbon deposits. Once filtered the pressurised oil collects in a reservoir called a gallery from which it is directed through holes in the engine to the cylinder heads where it lubricates the pistons. It also lubricates the bearings in the camshaft. Oil is needed to lubricate moving surfaces to prevent them from damaging each other, to reduce friction and, therefore, to increase efficiency. To do this effectively the oil has to maintain pressure. An oil pressure gauge is fitted to some cars to allow the driver to monitor the pressure but even if your car does not have a pressure gauge it will have a warning light that comes on when the level of oil in the gallery is not sufficient to lubricate the engine. Lubrication also makes the engine quieter.
The level of oil should be checked regularly and a complete oil change is needed from time to time. Motor oil also helps to clean the engine. When it is made the oil has extra chemicals added to act as detergents and corrosion/rust inhpreibitors. It also uses additives to change the viscosity of the oil. Over time these additives become depleted and need to be replaced. Exactly how often will depend on your car. Consult with your owners manual for exact information. The type of oil used is very important. A cheap imitation of a well-known brand might not contain the necessary additives. To avoid problems stick to motor oil is approved by the American petroleum Institute. Cheap oil changes might save a few dollars in short-term but in the long term they can be costly.