Generally, a lubricant is selected with the minimum viscosity necessary to provide adequate separation of the working elements of the bearing. Excess viscosity will result in friction and heat generation within the bearing. Almost all industrial lubricants are based on animal, vegetable or mineral oils. Mineral oils, usually petroleum based, are graded light, medium and heavy, according to their viscosity. Machine, turbine, engine, spindle, compressor, cylinder and gear oils are examples of specifically designed mineral oil derived lubricants.
Greases are sometimes more convenient to use than liquid oil lubricants.
They are, however, in high speed or severe duty applications, somewhat of a compromise. They do not provide the cooling effects of an oil-type liquid lubricant. Greases are solid to semi-solid products that contain thickening agents, usually in the form of metallic soaps. The remainder is the thickening agent. Only the oil portion lubricates.