The most important thing oil does in your engine is lubricates moving parts such as piston heads against cylinder walls and bearings against the crankshaft, so that these parts do not wear out because of friction and heat. Because the oil is pumped around the engine and kept under pressure, the oil is sandwiched between these components and stops them from coming into contact with each other. This is one reason why the pressure of the oil in the engine is so important. If the pressure is not great enough it cannot form a protective layer of oil between these parts. The pressure is needed to resist the forces trying to push the components together and the oil pressure gauge is how the car let you know when there is something wrong with the oil system. (For more details see the Oil Pressure Gauge Guide blog.)
Oil also perform some other important tasks in the engine it helps to cool it down. As the oil circulates it, for want of a better word, ” absorbs” some of the heat energy in the engine. As the oil collects again in the oil pan or sump, it is cooled somewhat by the air flowing around the engine as the car moves. Unfortunately this is not enough cooling to counteract the extreme heat in the engine so other water based cooling methods are needed in addition to the oil.
Finally oil helps to clean the engine. Over time the oil in your car becomes contaminated and starts to form a slimy black sludge which sticks to the engine, inhibits cooling (by making the engine hotter), reduces the flow of oil and starts to corrode the engine. Many engine oils contain detergents to remove and reduce the effects of this sludge but there is no substitute for regular oil changes. If you have any questions about your oil system you should consult your owner’s manual or a qualified mechanic.