First principles: No. 6. Friction.

A LUBRICATING oil may have several duties to perform in addition to acting as a lubricant. It may have to act as a coolant and carry frictional temperature to where it can be easily dissipated to atmosphere, it may have to operate hydraulic or other mechanically operated control or governor gear, it may have to act as an insulating oil, it may have to effect a seal against escape of exhaust gases or a seal between piston and rings. However, the main duty of any lubricant will be to reduce friction between moving surfaces. In order to obtain a clear understanding of how a lubricant can reduce frictional resistance, it is necessary to know the fundamental theories concerning friction and to know the rules of friction. Friction obeys certain rules whereby its value, or its powers of resistance, can be calculated sufficiently accurately for most designer's purposes.