Hydraulic oils, when put under pressure do not change their density and are known as incompressible fluids. Because they don't compress, forces are transmitted through the fluid with minimal losses. Both water and hydraulic oils are incompressible, but hydraulic oils are used because they also have the property of lubricity to aid in the sliding of major components, sealing, and reducing corrosion. This property allows them to be pumped into cylinders, like the one seen below and with the opening and closing of their outlets, enables the extension and retraction of the rod with the yellow eye.

Cutaway view of a hydraulic actuator

Applications vary somewhat, but the fundamentals of a hydraulic system are pretty basic.

• Hydraulic fluid tank connected to a hydraulic pump
• Pipes and hoses to connect the pump to the hydraulic cylinder and rod assembly  
• Valves and by-pass devices to control the high pressures and thus movement of the rod

Once the movement and work is done, the rod returns to its initial position and the fluid is recirculated back to the tank at low pressure. The motor to drive the pump can be electric, internal combustion, or diesel.

Schematic of a simplified hydraulic system

The Original Equipment Manufacturer will always specify oils for use in their equipment. Brand names can be confusing but the common design characteristics are viscosity, the operating temperature range; online tools and charts can help make these comparisons across brands easy. Hydraulic oils also need to: keep their viscosity over the operating temperature range, stay liquid (that is, they should not boil, freeze, or generate foam or solids under operating conditions), and should not be corrosive. To get into the details, visit oilviscositychart.com. for more information on hydraulic and other industrial oils.

In some cases it's helpful to send in a sample of your oil to get tested and receive a professional analysis. Up front it is important to choose the oil to meet your machines requirements. Key oil design characteristics for oils are its viscosity, operating temperature range, and essentially operating velocity, which can be easily determined at oilviscositychart.com. And later at service intervals it is important to sample and analyze your oil which can be very indicative of remaining service life. Equipment varies, but trace elements of additives, wear metals, and other contaminants can be determined by a good analysis to guide maintenance.

There is a lot to consider for your particular application as seen above. Synthetic and specialty hydraulic oils will be more expensive but tend to perform for a longer maintenance cycle. Comparable and more standard hydraulic oils range from $10.00 to $13.34 per gallon for a 5-Gallon Pail. Liquids are heavy, so if you can find a local supplier, you can skip a large shipping fee. Expect to pay more for smaller quantities, and generally less for larger quantities. The main ingredient is refined oil and it is derived from produced mineral oils which are trading at around $42/(42-gallon Barrel) or about $1.00 per gallon. As you can see the premium for refining, packaging, marketing... and delivery is high