01/09/2026 by Jay Hatton
Whether you’re using hydraulic oil made by Houghton or gas engine oil from Total, oil analysis is a critical process to ensure fluids are fit for purpose.
As a result, the ability to understand oil analysis results is equally vital and can inform decisions concerning preventive maintenance processes.
Fortunately, the skill of interpreting reports can be developed with a little investment in certification and training, keeping costs low for companies. Here, we explore oil analysis and how to read reports.
Viscosity
Considered the most important attribute of a lubricant, viscosity is understandably the most common test conducted. This test effectively measures how resistant a lubricant is to flow at a set temperature. When a lubricant doesn’t have the correct viscosity, it cannot carry out its role.
Oils are identified by an ISO viscosity grade (VG). This refers to the lubricant’s kinematic viscosity at a temperature of 40 degrees Celsius (C). Classification as a certain ISO grade requires the oil’s viscosity to fall within minus or plus 10 per cent of the grade.
To interpret the viscosity section in a report, check if the measured kinematic value at the established test temperature falls inside the target range for the oil put in. If the viscosity is substantially higher than the acceptable range, the lubricant is likely experiencing oxidation, heavy contamination or nitration, causing it to thicken. If the viscosity has fallen below the grade, it has typically been diluted by incorrect top-off oil or fuel, or has experienced extreme thermal breakdown.
Particle count and the ISO cleanliness code
How concentrated wear particles are distributed in oil is an indicator of possible component problems. As a result, an analysis must measure a wide range of contaminant and wear particles.
While some wear creates extremely small particles, other types produce larger particles that are physically visible in the oil. Regardless of size, particles allowed to enter oil tend to cause serious harm to mechanical systems and their components.
Particle count analysis is performed on a sample of the oil used in a system. A particle count test reports the quantity and size of particles and represents solid contaminants present in the fluid. The ISO cleanliness code is used to work out solid contamination levels in both used and new oils.
If the system’s cleanliness level falls short of the desired target, action is recommended for remediation.
Particle counts are usually listed in six size categories: larger than 4 microns, larger than 6 microns, larger than 14 microns, larger than 25 microns, larger than 50 microns and larger than 100 microns. Reported values offer a clear understanding of the solid particles in the oil being used in a system.
Wear metals
To interpret the wear metals section in an oil analysis, match the metal elements that are listed in parts per million (ppm) with the internal system components they represent.
There are several key wear metals commonly listed associated with different parts. Iron originates from cast-iron or steel components like cylinder liners, gear trains, crankshafts and camshafts, and high levels indicate general ferrous wear.
Copper and lead typically point to sleeve wear or wear on soft-metal surfaces of bearings. Copper may also indicate wear to an oil cooler core.
Aluminium wear comes from main engine bearings, pistons or turbocharger bearings.
Chromium wear metals originate from hard-faced components, cylinder liners and piston rings.
TAN and TBN
The Total Acid Number (TAN) listed in an oil analysis report is designed to measure how acidic a lubricant has become and is expressed in mg KOH/g (milligrams of potassium hydroxide per gram of oil).
As a lubricant degrades from oxygen, use and heat, it starts to build up acidic by-products, and as a result, its TAN increases. A high TAN indicates that an oil is becoming corrosive and unstable, which can result in damage to internal metal surfaces within systems.
A baseline comparison is critical because brand-new oil does not always have a TAN of zero. You must compare the current TAN against previous samples or the new oil baseline instead of using a universal number.
In broad terms, when reading the numbers in a report, a low TAN indicates that the lubricant is healthy and has not experienced breakdown or severe levels of oxidation. However, a rising trend shows that the oil is either aging, suffering additive depletion or coping with high operating temperatures.
In many oil analysis reports, the TAN is examined alongside the Total Base Number (TBN). This number measures the lubricant’s alkaline reserve (how well it can fight acid). If an increasing TAN intersects and exceeds a decreasing TBN, it clearly indicates that a lubricant is no longer suitable for active service and must be changed.
Water
The confirmed presence of water in an oil represents a severe threat to the mechanical equipment. An inferior lubricant, water promotes corrosion and rust (iron oxide) on the metal surfaces of components.
When dissolved in oil, water produces oxidation and diminishes the oil’s ability to handle loads.
Water contamination is also a root cause of additive deterioration and depletion. In any form, however, water results in several unwanted outcomes, including high operating temperatures, accelerated wear and increased friction. If it is not removed, it can cause premature failure in components.
The most used method of analysing water levels in a lubricant is the coulometric moisture test developed by Karl Fischer. When reviewing test results, you must remember that low water levels in an oil are usually caused by condensation, while the presence of higher levels indicates water is entering the system. As a rule for most mechanical systems, water levels should not be higher than 500 parts per million.
Some of the common sources of water ingress include internal leaks in water jackets and heat exchangers, external contamination in seals, reservoir covers and breathers and condensation.
Oxidation
In an oil analysis report, oxidation measures the thermal breakdown and chemical aging of a lubricant. It displays how much a lubricant has reacted with oxygen while in service. It also shows if byproducts are present.
High oxidation levels mean that organic acids, sludge and varnish are forming in the oil. An increase in viscosity is caused by oil molecules that have oxides and have become stuck together.
Labs typically report oxidation utilising Fourier-transform infrared spectroscopy (FTIR) in units of absorbance per centimetre (Abs/cm) or using a scaled index number. A baseline of 10 or less is indicative of fresh mineral oil. A sample of new oil usually has a very low value to start with, which is often under 10. If the index number is between 20 and 30, it indicates normal wear and moderate aging for an oil that is getting close to the end of its standard service lifespan.
A high number in the range of 30 to 40 or greater indicates that a lubricant is thermally stressed, severely aged or has reached the end of its useful service life and requires changing.
Additive depletion
Interpreting additive depletion using an oil analysis report involves comparing current levels of TANs and TBNs in lubricants and specialised test markers to clean baseline data for oils in use.
Elemental spectroscopy (often referred to as additive markers) monitors active elements like phosphorus and zinc (anti-wear additives) and magnesium and calcium (detergent additives). You should check for downward trends from new oil baselines, as falling levels indicate additives have been consumed or are dropping out of suspension.
As the TBN measures an oil’s alkaline reserve that is available to neutralise acids, a substantial drop (below 40 to 50 percent of the fresh oil value) indicates severe additive depletion.
Warning and critical limits
An oil analysis report will also highlight warnings and critical limits. You can review the laboratory’s colour-coded alert columns or severity flags next to each itemised measured parameter. Labs set thresholds founded on information like Original Equipment Manufacturer (OEM) specifications and baseline fluid data.
Trends versus one-off results
When reading and interpreting reports, it is important to understand the difference between one-off results and trends. While a single high reading suggests a resample is required, an upward trend across several samples is a strong sign of active component wear that requires immediate correction.
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What is the difference between oil and grease?
From Castrol to Kluber, many lubricant manufacturers produce both oil and grease products.
What is the ISO cleanliness code?
The non-governmental independent global group, the International Organization for Standardization (ISO), has established a cleanliness code for use in reports regarding industrial oil analysis.