How to determine the best choice of hydraulic fluid to use for your equipment or system.
Although many manufactures of hydraulic fluid provide various grades of detergents and oxidation stability in the fluids they produce, the single factor that should weight the heaviest in determining the best choice of fluid to use is viscosity, otherwise known as the fluid's weight. Viscosity is also most often expressed as the fluids ISO number or grade.
Common ISO grades for use with hydraulic systems include 32, 46 and 68. Sometimes, 10, or 100 weight is mentioned as well. I would like to point out that for basic operation of any hydraulic system without concern to long-term effects of the systems life, 10W-40 automotive engine may be used, providing it does not contain detergent additives, and these additives used in engine oil can cause corrosion and cavitation. This occurs because the detergents can cause water that has condensed into the system to emulsify, preventing it from being separated from the oil.
It is recommended however, that a proper hydraulic fluid be used, and chosen based on its ISO grade. These fluids do not contain harmful additives that can cause damage to components, and are designed to work within a specific temperature range.