• Hydraulic oil is made mostly of mineral oil and water.Hydraulic pipes from an industrial vehicle image by JCVStock from Fotolia.com
  
  Hydraulic oil is an oil used to transfer force in hydraulic systems. These systems pressurize a fluid and use it to perform work, just as pneumatic systems use gas. The fluid is typically contained within a watertight piston strong enough to focus the pressure of the hydraulic oil in a certain direction. Many different types of oil are used, but they have the same basic ingredients, allowing them to perform for long periods of time without causing significant wear to the system itself.
  
  

Mineral Oil

• Mineral oil is one of the most abundant components of hydraulic oil. It is a very common petroleum by-product that is produced as petroleum is refined for its more complex elements. Mineral oil lacks the toxicity of other petroleum products, but it is still made of hydrocarbons and can act as an excellent lubricant. This allows the hydraulic system to move the fluid smoothly, gaining more energy and accuracy.
  
  

Water

• Water is the base hydraulic fluid. For many years most hydraulic liquids were composed mostly or entirely of water, which had well-known pressure properties and was the cheapest option. However, water also has several problems: it is not the strongest hydraulic fluid and it rusts the systems that use it. Hydraulic oil tends to be around 35 to 40 percent water (CDC).
  
  

Emulsifiers

• Emulsifiers are a specific type of surfactant. They help make the hydraulic oil more uniform in mixture, keeping the oil and water (usually two substances that try to separate) mixed together by pulling their molecules into small groups. There are only a small amount of emulsifiers in hydraulic oil compared to mineral oil and water.
  
  

Additives

• Additives are the fourth and most diverse group of hydraulic oil components. Manufacturers add these compounds to help hydraulic systems perform more efficiently and endure higher pressures than otherwise possible. Organic sulfur, phosphorus and chlorine compounds are all common additives.