"A customer of ours is using an external engine oil filtration device designed to take out contaminants down to the 1- to 3-micron range while not affecting the additive package. The customer now believes he can extend the recommended oil change interval from 250 hours to 1,000 hours. However, if you take out all the normal wear particles, how can you determine or trend the wear in the engine."

By sampling after the pump and before the filter, you can still see an increased rate of wear generation with oil analysis. By reducing the background level of wear particles (noise), it is comparatively easier to detect the abnormal generation of wear particles assuming, of course, that the sample is drawn after the pump but before the filter.

You will also need to set alarms carefully, using statistics to derive level limits and rate-of-change limits. The benefit of engine life extension associated with polishing the oil is considerable and will easily warrant some efforts to ensure that oil analysis can still the generation of abnormal wear.

I would advise your client to make sure that any decision to extend the oil drain interval is backed up by oil analysis (oil properties, contamination and wear debris monitoring). It is unwise to arbitrarily extend oil drains unless the decision is supported by data. Excessive particle contamination level is just one reason why we may change the oil.

The best way to tweak your oil change interval is to make actual assessments of its condition and remaining useful life. Unlike routine oil analysis, the types of tests you might want to select could be more similar to what you see on your lubricant's spec sheet.

Begin by talking to your lubricant supplier about which performance properties would need to be evaluated. These could include oxidation stability, rust/corrosion protection, air release and foam stability, demulsibility, anti-wear protection, viscosity index (VI), silt particle concentration and other essential performance properties. Expect the cost of these tests to run several hundred dollars. However, you only have to run tests on samples from a few representative machines.

Obtain the sample just prior to the currently scheduled oil change. Send this sample along with a sample of the new lubricant (for baselining purposes) to a lab that can perform ASTM performance tests. You may need to obtain several hundred milliliters of fluid of each sample.

Once the tests are complete and an assessment made on remaining useful life, a decision can then be made on whether the oil change interval can be shortened or lengthened. A reasonable safety margin needs to be included in the decision. Once changes in the drain interval are made, monitor the oil carefully.

While fluid analysis is the best way to safely determine an optimal drain interval, the appropriate testing should be done by a reputable laboratory that uses reliable testing methods and produces quality results you can trust. Accredited laboratories are required to prove the accuracy of their results on an on-going basis or risk losing their accreditation.