Though full of helpful information, oil analysis results are best used to indicate what to look for, not absolute indicators of, problems.
P. Abelson Though full of helpful information, oil analysis results are best used to indicate what to look for, not absolute indicators of, problems.

I went to the doctor recently for my annual checkup. Before the office visit, I had blood work done to see if my medications were effective, if there were any interactions, and if I had any ailments that were starting to make their presence felt. As I waited to be tested, I realized that modern medicine had caught up with vehicle technology.

Or is it the other way around?

For more than 25 years, fleet managers have looked at used fluids to determine the health of a component and to project future wear. Just as blood analysis indicates how your organs are functioning, used oil and coolant analysis indicates how an engine’s doing. Analysis indicates how much life remains in the oil by measuring reserve alkalinity: the total base number (TBN) or acidity expressed as total acid number (TAN). As long as TBN exceeds TAN, the oil is useable. The difference between TBN and TAN indicates roughly how much longer the oil can be used.

Viscosity also determines remaining useful life. As oil is used, polymer viscosity improvers undergo mechanical shear. When oil loses viscosity to the point that film strength is diminished, oil must be changed.

Dirt and contaminants affect oil performance. Changes in the amount of silicon in oil could indicate if dirt is present. This is especially important when vehicles operate off paved roads, but silicon also comes from other sources, such as RTV gasket material.

Soot in oil is a product of improper or incomplete combustion or leaking exhaust gas recirculation devices and also contributes to component wear. Essentially incompletely burned hydrocarbon fuel, it can be an abrasive. As such, it acts like a lapping compound carried by the oil, wearing all moving parts.

Identifying worn parts

Oil analysis also identifies parts that are starting to wear. After checking for viscosity, soot, dirt, and TBN/TAN, check metal levels. Alone or in combination, they indicate which parts are wearing out.

Iron and chromium in a diesel engine could indicate piston ring or turbocharger shaft wear. High iron but normal chromium levels could indicate cylinder liner, valve train, or crankshaft wear. Lead, tin, and copper often indicate bearing wear. Or they could be from oil coolers or turbocharger bearings. See American Trucking Associations Technology & Maintenance Council Recommended Practice 1420.

Interpreting analysis results

There are two ways to analyze and interpret fluid analysis results: set limits and trend analysis. When an oil supplier or independent laboratory reports that oil is good to use or is out of specification, they use set limits. For every element or property they examine, they have set limits for warnings and alert values. For example, if the warning level for iron is 100 parts per million (ppm), 99 or less indicates the oil is good; 100 or more generates a warning message.

Trend analysis also uses set limits, but once trend lines are established, deviations from those trends generate alerts. Let’s say iron, measured at 15,000-mile intervals, reads 52 ppm, 48 ppm, 55 ppm, and 88 ppm. The last reading wouldn’t affect a set limit report, but because it deviates sharply from the trend the deviation would trigger an alert to further investigate.

For many years, the City of Madison, Wis., didn’t analyze oil because most manufacturers didn’t encourage extended oil drains and the process didn’t fit the city’s operations. “That’s all changed,” says Fleet Service Superintendent Bill Vanden Brook, CEM. “Manufacturers allow and even encourage them, so we do it. We sample engines at every oil drain, transmissions twice a year, and hydraulics annually. We’ve caught problems early and saved quite a bit in repairs. It’s also a lot cheaper today. Analysis costs less than a gallon of oil, sometimes less than a quart.”

Paul Abelson ( is a former director of the Technology and Maintenance Council (TMC) of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.