The primary function of a hydraulic fluid is to convey power. The properties and ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base.
Because industrial hydraulic systems operate at hundreds to thousands of PSI and temperatures reaching hundreds of degrees, severe injuries and death can result from component failures and care must always be taken when performing maintenance on hydraulic systems.
Multi-grade or Mono-grade
It does not matter how good the other properties of the oil are if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system. If the hydraulic system is required to operate in freezing temperatures in winter, and tropical conditions in summer, then it’s likely that multi-grade oil will be required to maintain viscosity across a wide operating temperature range.