The insolubles test measures the total insoluble materials in an oil sample, that is, all solid or liquid materials that are not soluble (won't mix) in oil. Virgin oil shouldn't have any insoluble materials in it. When it occasionally does, the most we normally find is a trace level. The insolubles in virgin oil are from the normal oxidation process of the oil, which leaves free carbon in suspension when oxygen forms with hydrogen (oil is a hydrocarbon).
The insolubles test is a centrifuge method. A measured volume of oil is mixed with a heated solvent, agitated, and spun at high speed. Insoluble materials collect at the bottom of a tapered glass tube and can then be quantified. The insolubles test is a fair measure of how fast the oil is oxidizing and receiving contaminants, and how effectively the system's oil filtration is functioning.
Industrial oil normally contains very low insolubles due to the few and relatively mild heat cycles the oil experiences (heat cycles accelerate the oil's normal tendency to oxidize). Further, oil filtration on industrial machines may filter particles as small as 2 to 10 microns, keeping the oil pristine for a very long time, often years.
Automotive and aircraft oils however, suffer the most difficult environmental problems of all types of oils we analyze. They regularly receive blow-by products from the combustion process. They suffer extreme heat cycles. Any contaminant in the oil will accelerate the oxidation process, causing insoluble materials to increase. Engine oil needs to be changed regularly due to all of the above.
Excessive insolubles can form in an engine oil if the oil: is running hot, is receiving more than a normal amount of contamination, is suffering more (or more severe) heat cycles than is normal, is being run longer than a typical use cycle, or, on the other side of the coin, if oil filtration is marginal or relatively ineffective.
If we found no contamination in your oil and your change intervals are normal, we often mention a problem at oil filtration as a possible cause of higher insolubles. Insolubles may be forming because your oil change interval is too long for the condition of the engine. Your oil filter may be inferior. It is possible the oil filter bypass valve has relived if the filter is becoming restricted. The filter system bypass may also open upon unusually cold starts when the oil is too thick to pass through the filter media. Once the bypass relieves, the filter is effectively out of the system.
The insolubles test is a fair measure of several possible problems in your engine. One high reading needn't be a cause of concern. Several high readings in a row merit investigation of what the problem may be.