Hydraulic oil and hydraulic fluid are terms that are sometimes used interchangeably, but they are not necessarily the same. While hydraulic oil is a fluid, hydraulic fluid can also consist of other fluids, including plain water, water-oil emulsions and salt solutions.
Because hydraulic oil is flammable, it not suitable for use close to an ignition source. Under high pressure, oil spray can ignite. Hydraulic fluid was developed after World War II as a solution to the problem.
Hydraulic oil consists of oils and additives designed to transmit power while acting as a lubricant and coolant. The oil is effective in a wide range of temperatures and reduces wear, rust and corrosion in equipment used in industries like agriculture, mining and construction.
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Hydraulic fluid is used for many of the same reasons as oil but has wider use in automobile systems like automatic transmissions, as well as power brakes and steering. Aircraft systems also require hydraulic fluids. Viscosity is of paramount importance in choosing a fluid.