When changing oil or lubricants in large equipment that has a long run time between changes, retain a small sample in a clear container to have for baseline comparisons. A color change may not be noticable unless a side-by-side comparison can be made. Also, there may be slight changes from batch to batch or from different suppliers. (Submitted by Ken Waugh, Sr Engr Tech, Chaparral Energy)

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Book Bits: Estimating Oil Drain Intervals

From "Lubrication Fundamentals"

Drain intervals can vary considerably depending on the engine and service. For example, a large diesel engine in central station use, with a relatively large crankcase oil supply, may operate for thousands of hours between oil changes. Such engines are usually in good adjustment, temperatures are moderate, and the contamination rate is low in comparison to the volume of oil in the system.

On the other hand, a passenger car engine may require an oil change every few thousand miles. Such engines are physically small with relatively small crankcase capacity, and operate under conditions conducive to rapid oil contamination. Not only are load factors often low, but these engines may be engaged mostly in short runs and start-stop service, as well operating over wide ranges of ambient temperature, all of which favor the accumulation of oil contaminants and the risk of deposits.

The presence of an oil filter does not necessarily permit an extension of the oil drain interval. Filters do not remove oil-soluble contaminants and water, which are important factors in deposit formation. Regular filter changes are, however, important in keeping the filter operable so that it can perform its function of removing insoluble contaminants from the oil.

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QUESTION: In lubrication terms, what is the "Right Principle"?

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"What happens to the additives in oil over time, and is there any way to replenish them?"

Over time, additives are depleted performing the function for which they were intended, degraded by hydrolysis, mechanical shearing, condensation settling, water washing, particle scrubbing, etc.

The rate of depletion or degradation depends upon the application and the environment. In particular, heat, pressure, shear rate, fuel sulfur, soot, dirt, water, aeration and the presence of catalytic metals (copper, iron, etc.) affect the rate of depletion.

Regarding replenishment - whenever you top-up a system, you are replenishing additives. Likewise, one can perform a partial drain and replacement (often referred to as bleed and feed).

A bleed and feed can work if the base oil is not degraded. If the base oil has been degraded, adding new oil is analogous to sending a healthy person into a room full of sick people with the hope that his or her good health will be contagious - it doesn't work that way. The additives in the new oil might be compromised within the first hours of use, leaving you right back where you started.

Casual addition of additives into a formulated oil can be dangerous and should be avoided. When in doubt, consult your lubricant supplier.

Drew Troyer, Noria Corporation

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