Motor oil is a lubricating liquid used in car engines to reduce friction between the moving parts inside a combustion engine. Principally it prevents moving parts rubbing against each other as they convert heat energy from the burning of gas and air into kinetic energy that moves the car forward. Without oil forming a cushion between the parts for them to glide over these components would quickly become damaged from the wear and heat caused by the friction of their movement. Oil also has less important roles to play in keeping the engine cool and keeping the engine clean. Both of these are important but lubricating is the main purpose of motor oil.
It’s important that the motor oil is kept at the manufacturer’s recommended level of pressure. If the pressure falls, as indicated by your oil pressure gauge, oil loses its ability to lubricate. This is because the oil it needs to provide some resistance against the forces which try and push it away when two moving parts meet. If there was no pressure the oil would be swept away as the moving parts touched. Imagine a pool of water on the workbench, when you flatten your hand against the water it is easily dispersed by the pressure of your hand. But if the water had nowhere to escape to it would take a considerable amount of force to touch the bench. (You can learn more about oil pressure and oil viscosity at the oil pressure gauge guide).
Finally, oil cool and cleans. Engine oils containing chemical additives which act as detergents to clean inside the engine and, because it covers the internal surfaces, it inhibits rust caused by oxidisation. It is important to change your oil on a regular basis. The performance of your car really is affected by the quality of oil therefore I would not recommend a cheap oil change to save money.
