"A customer of ours is using an external engine oil filtration device designed to remove 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. My question is if you remove 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, we 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 investment 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.