Today's diesel engine lubricating oils are highly dispersant and typically carry several pounds of solid combustion contaminant in suspension - little of which is filterable. Since the average full flow filter is capable of holding less than a pound of solids, filter life and capacity are no longer directly related. Field experience has demonstrated at least six distinct methods by which filters plug. This data indicates that filter life is simply a function of the type and amount of contaminant present in the crankcase and the oil's ability to handle or tolerate the contamination. However, oils differ greatly in their ability to do this. Field experience also suggests that the current API Classification tests may be inadequate in some areas to determine lubricating oils' suitability for diesel engine service. Finally, filter plugging should be looked at as a symptom of engine or oil problems, but not as a problem itself.