From the book "The Practical Handbook of Machinery Lubrication":
Keep hydraulic fluids cool. (The bulk oil temperature at the exterior of the reservoir should never exceed 60°C).
Keep hydraulic fluids clean. (There is general agreement among hydraulic experts that 75 - 80 percent of hydraulic failures are caused by fluid contaminated with dirt, wear particles and other foreign material. In today's high-pressure systems, clearances between wear surfaces are very small, making contamination control critical).
Keep hydraulic fluids dry. (Water and condensation content should never exceed a maximum of 1000 ppm, depending on the system design).
Immediately repair fluid leaks. If oil can escape, dirt and dust can re-enter the system. (A fluid leak of one drop per second is equal to 400 gallons in a 12-month period.)
More information about "The Practical Handbook of Machinery Lubrication"