The essential purpose of a lubricant is to reduce friction and to minimize wear. This object is achieved by preventing the physical contact of moving surfaces by maintaining between them a thin layer of a suitable fluid, the lubricant. The lubricant should adhere strongly to the boundary surfaces, so that when they are in relative motion, a film of lubricant is “dragged along” between them, forcing them apart and keeping them out of physical contact with each other.

The maintenance of this layer during actual movement of the surfaces depends on a property common to all fluids, namely, viscosity. Viscosity is a measure of the resistance of liquids to shearing forces; for instance, an oil which pours out of a spout slowly is usually termed a “thick oil”, while one which pours easily is described as “thin”. It is more correct to speak of these oils as having “high” and “low” viscosities respectively. In general, the higher the viscosity of the fluid used, the easier it is to maintain a film during movement of the bearing surfaces. But because a high viscosity means a high resistance to shearing forces on the lubricant itself, more power is needed to maintain the desired relative motion of the surfaces.