what contaminants are present in hydraulic oil
Contaminants of hydraulic oil are broadly defined as any substance that impairs the proper functioning of the fluid. Air fits this definition and therefore when air becomes entrained in the oil, corrective action is required to prevent damage to both the oil and the other components in the hydraulic system. Air can be present in four forms: Free air - such as a pocket of air trapped in part of a system.
What gets into oil. Water is probably the most common chemical contaminant in hydraulic systems and condensation the most likely source. A system run in hot, humid environments ingests air containing water vapor, which then condenses upon cooling.