Hydraulic power transmission naturally lends itself to a wide range of purposes where multiplication of force is required, or where accurate and dependable control gear must be provide. The immense forces that may developed by the application of hydraulic  principles and precision and flexibility of hydraulic controls, could in many cases be provide only with extreme difficulty by any other means. There is therefore, an almost endless variety of hydraulic gear, but the hydraulic media employed necessarily have many features in common.

The obvious prime requirements of an hydraulic medium are, that it should be relatively incompressible and sufficiently fluid to permit efficient transmission of power. These stipulations alone are met by many liquids and in fact, may have been employed more or less satisfactorily in the past. But conditions of service in modern hydraulic machinery are so exacting that few liquids are able to meet their complex requirements.

Water for instance, suffers from a number of disadvantages; it promotes rusting and possesses negligible lubricating properties. Furthermore it is liable to freeze in cold weather and to boil at a temperature which is low compared with that at which oil can be used. Emulsions of soluble oil in water are sometimes used in a partially successful attempt to overcome the two disadvantages mentioned earlier, but even such emulsions are, in general, quite unsuitable for modern applications. As hydraulic media, mineral oils are the most satisfactory products available in sufficiently large quantities and at reasonably low cost

Where hydraulic medium with greater fire resistance than mineral oil is required, various types of synthetic and water containing fluids are available. In particular, the fire resistant emulsion fluid a specially prepared water - in- oil type emulsion is gaining in popularity in this field. Extremely fine clearances are usual in modern hydraulic gear, and to avoid the excessive wear the hydraulic medium must possess lubricating ability of high order. Furthermore, the designs of hydraulic systems commonly involve the lubrication of associated mehanical gear by the hydraulic fluid. An hydraulic oil must therefore perform a dual function power transmission and lubrication. This must be considered in determining the viscosity, most suitable in an hydraulic oil for a specific application. Rapid and efficient transmission of power is less readily attained with the oils of high viscosity than with those of low viscosity. Greater internal friction with oils of higher viscosity may result in considerable loss of power and increase in operating temperature. In addition, the ability of the oil to maintain fluid film lubrication of working surface is affected by its penetrating and spreading ability. Oil of too high viscosity may fail in this respect also. On the other hand, the duties of the oil as lubricant require that its viscosity should not be too low; moreover, an oil of too low viscosity might fail to provide an effective seal at the clearance between moving parts - for example, failure of the oil to seal pump clearances would impair the efficiency of transmission. A further requirement of an hydraulic oil may is that change of viscosity over the service temperature range must not be too great. Some hydraulic systems, e.g. on machinery subjected to wide variation in ambient temperatures and on certain machine tools, require oils with superior viscosity temperature characteristics. Correctly chosen mineral oils are entirely satisfactory in this respect