The lubricating oil in a car is used for keeping the moving parts of the engine from grinding together and creating friction. A well-lubricated engine runs smoother, takes less gas to fuel and runs cooler.
When the car isn't in motion and the oil isn't being used, it pools in the oil pan, also known as a sump, which is located under the engine. The oil pan holds about 4 to 6 quarts of oil. Once the car starts, the oil pump sucks the oil up the pickup tube. The oil goes through the oil filter to filter out any dirt or debris that the oil picked up on its last trip around the engine. The oil then goes through spurt holes in places like the crankshaft and the bearings to keep these parts lubricated. When the car turns off, gravity takes over and the oil returns to the oil pan.
Changing the oil every 5,000 miles or so makes sure that the engine stays clean and runs smoothly since dirty oil doesn't do its job as well as clean oil. The dipstick in the car's engine compartment is used to check the level of the oil in the engine. Markings on the end of the dipstick indicate where the oil level should be.