Viscosity
Viscosity is the most important part of any hydraulic fluid. The viscosity grade determines the operating temperature range of the hydraulic system. Using the wrong hydraulic fluid in a hydraulic system will shorten the life of moving components.
Anti-Wear
AW hydraulic fluids contain anti-wear additives. Hydraulic systems that experience a wide operating temperature range, such as pumps, will work best with AW hydraulic fluids. Some AW fluids also contain viscosity index improvers. Hydraulic fluids containing improvers should not be used unless the equipment specifically requires it, as they can have a negative effect on air separation properties and demulsification, or seperation, of the fluid.
High-Density
HD hydraulic fluids are designed for systems bearing heavy loads. This type of hydraulic lubricant is thick and commonly used to lubricate gears and pistons. Lubricants like this are oil-based and help to prevent rust and wear due to metal-on-metal contact by providing a barrier between moving parts.