If you do not do regular oil changes on a vehicle, the oil gets dirty. When the oil gets dirty, it gets thick. The dirtier it gets, the thicker it gets, and over time, it will not be able to flow through the oil pump into the top of the engine. It will also block the oil galleys, preventing any oil in the top of the engine from returning to the oil pan.
Oil keeps the engine lubricated. There are many moving metal parts to the engine, and these moving parts create friction, which in turn creates heat, making the metal parts expand. If they expand too much, they will lock up on each other and the engine will no longer turn.
There are many types of oil. Most newer cars use 5W30 or 10W30. The first number denotes how thick the oil will get when it gets cold out and the second number is how thin the oil gets when it is hot out. The polymers in multi-weight oils determine this--so a 5W30 oil is a 5-weight oil that will not thin more than 30 weight when hot.
Oil should be changed every 3,000 miles or three months, whichever comes first. There are some cars that do not require an oil change as often, but most old-school mechanics still stand by the old time frame, as the oil in all vehicles tends to get dirty over time.
Most cars and light trucks hold 5 qts. of oil. This is enough oil for the car, plus a little extra for the oil filter. The oil dipstick on the top of the engine will show you when the oil is full. When adding oil during an oil change, add 4 qts., then check the dipstick. If the oil is not at the top of the crosshatches, add 1 more quart.
Check the oil in your vehicle at least twice a month. If you notice oil leaks get them repaired as soon as possible. Have your mechanic look for oil leaks when changing the oil, as the vehicle is up on the lift, so oil leaks are easier to find. Never let the car run without oil. If you change your own oil, start the car right after you finish the oil change. Make sure the dummy light on the dash goes out, or if you have a gauge, make sure the oil pressure comes up to at least 25 or 30 (newer cars or cars with rebuilt engines will be closer to 18-23kg of oil pressure).