A particle counter is a useful tool for maintenance and reliability professionals. It can provide both proactive and predictive benefits including information on:

• oil cleanliness level
• filtration performance
• abnormal wear conditions
• effectiveness of corrective maintenance
• problem isolation
• identifying the need for ferrographic analysis
• identifying use of dirty top-up containers or dirty new oil

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From the book "Lubrication Fundamentals":

In circulating systems, the lubricant, hydraulic fluid or metalworking fluid should be kept as free of contaminants as possible, first by preventing entry of contaminants into the system, and second by in-service purification during operation. Purification, depending on the nature and extent of the contaminants, may consist of continuous bypass or full-flow treatment. In many instances, a combination of the two methods is incorporated into the circulating system. In addition, large-capacity critical systems may use portable units for adding oil to the system or for periodic polishing, or independent purification units connected permanently to large reservoirs, or separate batch units to purify or reclaim drained lubricants for reuse.

More about the book Lubrication Fundamentals

Heat is often overlooked as a lubricant contaminant. However, heat can sometimes help control other contaminants in lubricants. Consider these advantages and disadvantages of hot running oil:

Advantages of hot running oil: good water shedding, lower foaming tendency, lower aeration tendency, improved particle settling rate, water vaporization, fuel vaporization.

Disadvantages of hot running oil: additive depletion, oxidation, thermal degradation , varnishing/coking, hydrolysis, loss of film strength (viscosity thinning), base-oil and additive volatilization, increased corrosion, seal failure, cost of synthetic base oil (to accommodate higher temperatures).

One of the most important size particles to control in lubricants and hydraulic oils is clearance-sized particles. Larger particles cannot get between moving surfaces and smaller ones pass right through. Clearance-sized particles usually get in and do the most damage. The "clearance" refers to the film of separation provided by the lubricant or hydraulic fluid.