Some common viscosity grades for hydraulic oil include 10, 32, 46 and 68. The viscosity of hydraulic oil is usually referred to as the fluid's International Standards Organization number.
Viscosity is the most important property of a hydraulic fluid. If the viscosity grade of the fluid doesn't match the operating temperature of the hydraulic system, the components of the system don't last as long as they should. The correct fluid viscosity is dependent on factors such as the starting viscosity of the fluid when at minimum ambient temperature, the maximum expected operating temperature of the hydraulic system and the optimum viscosity range for the individual components.
Hydraulic oils with high viscosity normally work well in reducing systems' operating temperature because they increase the volumetric efficiency. However, oils with a viscosity that is too high cause increased friction during startup, hence reducing the mechanical efficiency of hydraulic systems. A hydraulic fluid with the correct viscosity grade has a proper balance between the advantages of volumetric efficiency and the shortcomings of mechanical efficiency. The correct viscosity grade for a hydraulic system should be obtained from its manufacturer.
Automotive engine oil can be used in place of hydraulic oil without having long-term effects on the hydraulic system, provided it doesn't contain detergents or dispersants.