The lubrication system is very important in an engine. Failure in the system will produce a severe engine damage. Hence a periodic maintenance of the lubrication system is vital.

In principle, in a lubrication system, oil is drawn in through the oil pick up screen and then forced by the pump through an oil filter to the engine’s main oil gallery. Oil from the main gallery provides lubrication to the camshaft and cam bearings. The crankshaft receives oil at its mains bearings. The oil is pumped through drilled oil galleries from the main journals to the rod journals. The cylinder wall and piston are lubricated by oil thrown from the rod journals. The timing chain, the distributor, and the oil pump drive gear are all splash lubricated.

Oil is used to reduce friction, which depends on the roughness of the surfaces and the force exerted on the oil film. Under normal conditions, a breakdown occurs only when the engine is first started in the morning. The crankshaft rubs on the bearing until a wedge of oil is reestablished when pressurized oil reaches the bearing.

The engine oil is more than crude, it contains an additive package. In addition to lubrication, oil cools, cleans and prevents rust from forming inside the engine. It also helps sealing the piston rings against the walls of the cylinder.

The correct level of oil keeps the oil pick up screen submerged in oil under all operating conditions. If the oil level drops too low, serious engine damage can occur. The crankshaft bearings can be damaged or the piston can become scuffed. If the oil level in the crankcase is too high, the spinning crankshaft can dip into the oil, throwing it onto the cylinder walls in such quantity that the oil rings are overwhelmed. This can produce excessive exhaust smoke. In addition, the oil mixed with air does not provide sufficient oil pressure and can result in collapsed hydraulic lifters or even a broken crankshaft.

According to encyclopaedia  Britannica: Viscosity is resistance of a fluid to a change in shape, or movement of neighbouring portions relative to one another. Viscosity denotes opposition to flow. Oil normally thins when heated. As it becomes thinner, its viscosity becomes lower. An additive package containing polymers is blended into the oil. Polymers expand when heated to allow viscosity of cold oil to be more closely maintained as it warms. Oil viscosity is also critical on diesel engines with electronically controlled hydraulic fuel injection. The oil needs to flow at the correct rate if it is to operate the injector as designed.

Oils contain an additive package that can make up as much as one third of the volume of an oil container. Important parts of the additive package include:

  • Pour-point depressants that allow the oil to flow in very cold weather.
  • Corrosion and rust inhibitors that help the oil to stick to metal surfaces.
  • Antifoam additives that help dissipate bubbles that form as oil is moved around the engine.
  • Friction modifiers that reduce the friction between moving parts, resulting in less heat, reduced wear and improved fuel economy.
  • Oxydation inhibitors that prevent oil from thickening
  • Antiwear additives that combine chemically with engine metals during periods of high load