Interpreting an Oil Analysis Report

Used Oil Analysis (UOA) can be an extremely effective tool in the preventive maintenance process and can provide a degree of comfort when attempting to extend drain intervals. But what do those numbers mean? This is a general guide for understanding what the tests mean and what the numbers might be telling you. This will not pertain to every engine in every type of driving service but is intended as a guide. As always, follow your OEM recommendations for oil condemning limits.

It is critical to begin with a baseline analysis and develop trend data over time, and the same laboratory should be used for fresh and used oil analysis. It is important that viscosity of the oil stays “in grade”; a decrease of 0.5 cSt or more could indicate fuel dilution or shearing. A significant step change in viscosity indicates fuel dilution and may be cause for an oil change.

Wear metals—Iron (Fe), Copper (Cu) and Lead (Pb)—should be analyzed and compared to the fresh oil. It is completely normal for wear metals to be higher during engine break-in, but by the second or third drain any high levels should have come down. “Normal Operation” will show a linear increase (with respect to time or mileage) in wear metals over time, after the initial run-in. A non-linear or step-change in any of the wear metals indicates a drain is needed. Typically, Iron levels should be no higher than 300 ppm, and Lead and Copper should be below 30 ppm.

Total Base Number (TBN) should be compared to the fresh oil sample, and typically a change is indicated when the TBN reaches 50% of the fresh oil number. Some OEMs prefer to have the oil changed at a specific TBN (2.5 or 3.0 for instance); however, if wear metals remain linear then it may be possible to extend the drain to the lower limit.

A coolant leak can be catastrophic to your engine. Look for the presence of Sodium (Na) or Potassium (K) to indicate a coolant leak. Silicon (Si) levels over 30 ppm is an indication of dirt particles in the oil and should also be remedied immediately. Potassium and Sodium are sometimes used in additive packages and should be compared relative to the fresh oil.

Remember that a caution or red-flagged UOA parameter does not necessarily mean an oil change or engine tear down is necessary, just that there may be a problem and further investigation is recommended.​