by Robert Sanborn on Thu, Aug 10, 2006, who was a member of the STLE Society of Tribologists and Lubrication Engineers for 15 years.
This question often gets asked because the quantity needed to treat hydrualics can be rather large due to sump capacities. Most hydraulic oils contain no more than rust and oxidation inhibitors to extend the life of the fluid. Some use zinc/phosphorus as an anti-wear additive, however none address the cause of pre-mature pump failure and leaking fittings. Hydraulics are under great pressure and any contaminants coming through the breathers, or careless handling of adding further make up oil, can have devastating effects.
Hydraulic manufacturers adhere to a cleanliness code called ISO 4406, which describes the cleanliness level of hydraulic fluid for their equipment. The technology utilizes a laser to count and size the particles for normal cleanliness standards. Having dirtier fluid will shorten equipment life in half or less, and having extremely clean oil can increase equipment life by as much as 4 times. Mobile equipment is very susceptible because it is subjected to the elements of dust and moisture. Mobile equipment requires special attention