55 gallon drums of various ISO grades of hydraulic fluid. Common ISO grades for use with hydraulic systems include 32, 46 and 68.
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.