Understanding Lubrication and Cooling
Of the many duties oil has, one of the most important is lubrication. Lubrication works on a relatively simple principle. Oil pressure is used to form an oil film between moving parts, for example a crankshaft and its bearings. This space, called the oil clearance, is very small, about 0.002-0.003 inches. Movement of the parts also helps maintain the oil film. The rotating or sliding part is slightly suspended in the oil and does not touch the stationary part against which it moves. At this point, there is no metal contact or part wear. This is why most part wear occurs after a cold start; the engine has not built up sufficient oil pressure and all parts are not adequately lubricated.
The oil lubricates all moving metal parts, but it additionally cools the lower parts of an engine. Oil must remove about 40% of an engine's heat, whether it is water-cooled (has a radiator) or air-cooled (does not have a radiator). These oil-cooled parts include the crankshaft, main and connecting rod bearings, pistons, camshaft and bearings, and timing gear. The hot oil returns to the oil pan where airflow underneath the vehicle cools the crankcase and oil within. Oil temperatures in the crankcase may still reach 225° F. or more under normal conditions. Driving habits that place extra burdens on the engine would create more heat and raise your oil's temperature.