Oil plays an important role in engine operation, as you can see in this illustration. Oil is stored in the oil pan. From there, it is pumped by the oil pump through a network of passages to engine parts - including the cylinders. Once it is distributed throughout the engine, motor oil performs many valuable functions:

• It lubricates the metal surfaces of the engine, allowing moving parts to move freely and preventing metal from wearing away metal.

• It cools the engine by carrying away the heat created by the friction of moving parts.

• It keeps the engine clean by suspending combustion contaminants, dirt, and tiny metal particles so they can be removed with the oil and filter at each oil change.

• It forms a seal between the engine's piston rings and cylinder walls, allowing pressure to build in the combustion chamber and reducing leakage of water vapor and gasoline (called "blow-by") between the combustion chamber and the crankcase. Excessive blow-by causes power loss.

• It helps with start-up by remaining thin enough to permit the engine to crank - even at low temperatures. Yet it must be thick (viscous) enough to protect the engine at normal operating temperatures.