Hydraulic Oil Maintenance

Hydraulic Oils

When hydraulic oil turns from that golden honey color of new oil to a dark brown, does that mean it must be changed immediately? Is the system suffering from lost lubricating properties or gross contamination when this occurs, or is this a normal aging characteristic to be dismissed so long as the oil analysis results are within acceptable parameters? 

These types of questions are often asked whenever hydraulic fluid maintenance is discussed. Many people compare the oil in their industrial hydraulic systems to that of their automobile, assuming if the oil has turned to a dark brown that it must be changed as soon as possible regardless of how long it has been in service.