So when the car comes to a minor fault, which in no way affects the ride comfort, usually we ignore it.
It happens that neglected for a long time fault can lead to more serious damage.
Motor oil is a lubricant used in internal combustion engines, which power cars, motorcycles, lawnmowers, engine-generators, and many other machines.
This increases fuel consumption, decreases power output, and can lead to engine failure. Lubricating oil creates a separating film between surfaces of adjacent moving parts to minimize direct contact between them, decreasing heat caused by friction and reducing wear, thus protecting the engine.
In use, motor oil transfers heat through convection as it flows through the engine by means of air flow over the surface of the oil pan, an oil cooler and through the buildup of oil gases evacuated by the Positive Crankcase Ventilation (PCV) system. In petrol (gasoline) engines, the top piston ring can expose the motor oil to temperatures of 160 °C (320 °F).
In diesel engines the top ring can expose the oil to temperatures over 315 °C (600 °F).Motor oils with higher viscosity indices thin less at these higher temperatures. Coating metal parts with oil also keeps them from being exposed to oxygen, inhibiting oxidation at elevated operating temperatures preventing rust or corrosion.
Corrosion inhibitors may also be added to the motor oil.
Many motor oils also have detergents and dispersants added to help keep the engine clean and minimize oil sludge build-up.The oil is able to trap soot from combustion in itself, rather than leaving it deposited on the internal surfaces.
It is a combination of this, and some singeing that turns used oil black after some running. Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces.
Such particles could circulate in the oil and grind against moving parts, causing wear.