4 Oil Analysis Tests to Run on Every Sample

Oil analysis is one of the most powerful tools available for understanding machine health, but only when the right tests are run and the results are acted on in time. With dozens of available tests available, it can be tempting to overanalyze or, just as often, miss critical data that points to developing problems.

The four oil analysis tests outlined in this article form a practical baseline for nearly every sample. Together, they provide insight into wear, contamination, and lubricant condition, helping maintenance teams identify issues early and make informed decisions before failures occur. When these tests can be performed quickly and interpreted in context, whether through routine lab analysis or rapid on-site testing, the value of oil analysis increases by turning results into timely answers about machine health.

Elemental Spectroscopy

Elemental spectroscopy describes the test used to detect wear. There are approximately 30 different types of spectroscopy, however the most common types performed for oil analysis are inductively coupled plasma (ICP) spectroscopy, (widely used in large volume labs) and rotating disk spectroscopy (RDE) (widely used with onsite labs, or non-lab environments).

 Elemental spectroscopy describes a technique whereby an oil sample is excited to emit light, and the light intensity can be measured.  Light is measured in the visible and ultraviolet regions of the spectrum. The method of excitation to emit light  is the primary difference between ICP and RDE spectroscopy, although there are many other considerations why one method is used over another depending on the need.

Different elements produce different frequencies or colors. The intensity of the light emitted is directly proportional to the concentration of the element.

The elements are divided into three broad categories on the reports:

• wear metals, such as iron from gears
• contaminants, such as lithium, which indicate the presence of grease
• oil additives, like phosphorus, which is found in extreme pressure and antiwear additives. The most common elements tested by spectroscopy are in table 1

Element	Symbol	Found in
Iron	Fe	Gears, roller bearings, cylinder/liners, shafts
Chromium	Cr	Roller Bearings, piston rings
Nickel	Ni	Roller Bearings, camshafts and followers, thrust washers, valve stems, valve guides
Molybdenum	Mo	Piston rings, additive, solid additive (Mo-di)
Aluminium	Al	Piston, journal bearings, dirt
Copper	Cu	Brass/bronze bushes, gears, thrust washers, oil cooler cores, internal collant leaks
Tin	Sn	Bronze bushes, washers and gears
Lead	Pb	Journal bearings, grease, petrol contamination
Silver	Ag	Silver solder, journal bearings (seldom)
Silicon	Si	Dirt, grease, additive
Sodium	Na	Internal coolant leaks, additive, sea water contamination
Lithium	Li	Grease
Magnesium	Mg	Additive, sea water contamination
Zinc	Zn	Additive (antiwear)
Phosphorus	P	Additive (antiwear, extreme pressure)
Boron	B	Additive, internal coolant leak, brake fluid contamination
Sulfer	S	Lubricant base stock, additive

Table 1. Table 1 lists the most commonly occurring elements and their probable sources.

ICP Spectroscopy

ICP spectroscopy is an atomic emission (AE) procedure whereby the diluted oil is passed through an argon gas plasma.

The plasma is maintained at a temperature of approximately 8,000°C. In the upper region of the plasma, acquired energy is released as a result of the electronic transitions, and characteristic light emissions occur.   

Some elements can belong to more than one category. For example, silicon can be a component of wear debris (piston crown material), of the additive package (antifoaming agents), and of contaminants (dirt). Only by looking at a complete set of results is it possible to predict the source of the particular element.

In certain cases, labs use limits for contaminants. In the case of dirt, the limits in Table 2 are typically observed. Silicon is found in dirt, as well as grease, oil additives and silicone sealant. It is possible to see engines and hydraulic systems with silicon readings in excess of 100 ppm, yet these are still considered normal.

Test Class	Silicon Limit [ppm]
Engine	25
Drivetrain	100
Hydraulic / compressor / turbine	35 to 45
Automatic transmission	35 to 45

Table 2. Silicon Contamination Limits

Knowing where the elements may be found is useful, but it is more important to be able to determine the actual source as accurately as possible. Table 3 shows a few cases of wear and contamination and how they typically appear.

At this stage the importance of submitting sample information, particularly service meter reading, overhaul/replacement information and period oil in use must be strongly emphasized. The service meter reading and overhaul/replacement information tells the diagnostician what sort of wear rates to expect.

A new component can be expected to wear faster than a component in the middle of its life span because it “seats” into other wear surfaces. A component with high hours of service can be watched for increased wear as fatigue sets in.

Situation	Results
Dirt entry	Si and Al present, usually between 2:1 and 10:1. Watch the increase in the trend. Often accompanied by associated wear when present over acceptable limits.
Piston torching	Al and Siratio is 2:1. The Si originates from silicon carbide in the piston crown used to reduce the co-efficient of expansion. Seldom seen, as failure is usually rapid, and statistically there is little chance of getting a sample while occurring.
High Fe (alone)	Because iron is the most used construction material, sources are often varied. Consider valve gear and oil pump wear. Rust formation also produces high Fe.
High Si (alone)	Silicon by itself comes from a few main sources - anti-foaming agent additive, grease and silicon sealant. Usually seen in new/recently overhauled components. Usually can be ignored.
Top-end-wear (engines)	Characterized by increased levels of Fe (cylinder liner), Al (pistons), and Cr (rings). The presence of Ni usually indicates camshaft/cam follower wear.
Bottom end wear	Characterized by increased levels of Fe (crankshaft) and Pb, Cu, Sn (white metal bearings and bronze bushes). This wear is often precipitated by reduced base number (BN) or over-cooling as bearings become subject to corrosion from combustion by-products (acids). Fuel dilution often causes this too, but effects may be masked as diesel dilutes the oil and the wear readings.
Overheating (some cases) in engines	Increased additive levels (Mg, Ca, Zn, P, S) and viscosity. When light ends in the oil vaporize, the oil level decreases. Topping up increases the additive concentrations, as the additives themselves do not evaporate. Oxidation often not evident, as topping up replenishes antioxidants and boosts the BN. Often accompanied by Pb, Sn, and Cu as bearing wear can result from this situation.
Bronze bushing wear	Increased Cu and Sn levels. Cu:Sn ration usually approximately 20:1.
Bronze gear/thrust washer wear	Increased Cu and Sn levels. Cu:Sn ration usually approximately 20:1.
Internal coolant leaks	Increased Na, B, Cu, Si, Al, and Fe. Not all elements may be present. Often accompanied by increased Pb, Cu, and Sn as white-metal bearing wear often accompanies this. Water usually not evident, as it tends to boil off at normal operating temperatures.
Roller-bearing wear	Increased levels of Fe, Cr, and Ni, all components of race and roller materials. Increased Cu might result if brass/bronze cages are employed.
Hydraulic cylinder wear	Increased levels of Fe, Cr, and Ni.

Table 3. Common Wear Situation Indicated by elemental spectroscopy

The hours of use on the oil strongly influences what can be considered normal. An engine with 100 ppm Fe at 250 hours is likely to be healthy. The same reading at 10 hours probably indicates a serious problem. The chances of inaccurate diagnosis, particularly in the latter situation, increase without this information.

Furthermore, indicating a usage time value in months, especially for automotive components, is not particularly helpful - the vehicle could have been parked for that time or it could have had long daily commutes. For components without service meter readings, such as industrial gearboxes, an educated guess in months or years is better than nothing.

Limitations

Elemental spectroscopy is perhaps the most important  and useful test in used-oil analysis, and ICP spectroscopy the most widely used by oil analysis labs. however it is does have limitations. A key drawback is the size limit of the particles it can vaporize. It does not detect particles beyond the 3 to 5 -micron range.

While this limit does not affect the detection of most wear situations, there are times when it could be a problem. For instance, when a component fails due to fatigue, the wear particles generated tend to be larger than normal (this process is called spalling).

ICP does not detect these larger particles, so upon examining the trend, the iron level might seem to be dropping, even though the component is actually in trouble. Because of this limitation, other tests should be employed to provide an effective monitoring solution.

It is not always possible to use ICP analysis to measure the additive depletion of an oil. Take for example the detergent additive in an engine oil. This would reflect in the calcium value. If one measured the calcium level of both a new and a used oil, they would be similar, even though the detergent has been depleted in the used oil.

This is because the amount of actual calcium in the oil has not changed. What has changed is the form, or compound, in which the calcium exists. Before being “used”, the calcium was present in a compound with detergent properties. After being used, the calcium is still present, but now in an inactive form. Sometimes the depleted additive remnants settle out, and then ICP is useful, but apply judgment and experience when trending additive depletion.

There are exceptions to additive depletion-measuring limitations of ICP. Most notable is the case of borate-EP-containing oil contaminated with water. In this case, the extreme pressure additive containing the boron settles out of suspension and forms a sludge at the bottom of the sump.

If this precipitate is not captured in the sample, the boron level will read much lower than normal, indicating the oil is not fit for further use due to extreme pressure additive depletion. The converse, however, is still not necessarily true: if the boron level is correct, the oil may not necessarily still be fit for use.

Rotating Disc Electrode (RDE) Spectroscopy as an Alternative

Rotating Disc Electrode (RDE) spectroscopy is another atomic emission technique commonly used in oil analysis While the purpose of RDE is similar to ICP—measuring elemental concentrations in oil—the excitation method and particle-size sensitivity differ.

In RDE spectroscopy, a small volume of undiluted oil is introduced between a rotating carbon disc and an electrode. An electrical arc is generated, exciting the elements present in the oil and causing them to emit characteristic light. As with ICP, the intensity of the emitted light is proportional to the concentration of each element.

The SpectrOil 100 Rotating Disc Electrode Optical Emission Spectrometer (RDE-OES) is the eighth generation of the market leading RDE elemental spectrometer. It is widely used in commercial oil laboratories, on-site or trailer labs, as a proven means of precisely determining elemental composition in lubricating oil, coolant, light or heavy fuels, grease, and process water.

One of the key advantages of RDE spectroscopy is its ability to detect larger wear particles than ICP, typically up to 8 to 10 microns. Because the oil is not diluted and the excitation process is less restrictive with respect to particle size, RDE is more sensitive to coarse wear debris, which is often generated during abnormal or fatigue-related wear modes such as spalling.

However, RDE spectroscopy though very repeatable for used oil analysis, typically has lower analytical precision than ICP and is less sensitive to very low concentrations of dissolved metals. Additionally, the lower excitation temperature (compared to ICP) means a lower value for additive elements As a result, RDE is often best used as a field or screening technique, providing additional insight where ICP is either not available , or trends may appear stable or misleading.

When used together, ICP and RDE spectroscopy offer a more complete view of wear and contamination—ICP excelling at trending fine, dissolved metals, and RDE providing improved visibility into larger wear particles that may indicate emerging mechanical distress.

Particle Quantification Index (PQ or PQI)

In this test, each sample is passed over a sensor which measures the bulk magnetic content of the oil. Because iron is the major wear element in virtually all components, the PQI is really a measure of how much iron is present (ferrous density) in the sample, the amounts of other magnetic elements being negligible.

The PQI, unlike elemental spectroscopy is not size dependent - the bigger the number, the more ferromagnetic debris present,. What the PQI is communicating could be interpreted as a concept of mass per capacity or, in metric terms, something like grams iron per liter of oil. Newer devices such as FerroCheck  now measure the ferrous density to ppm concentrations, as per ASTM standards, making it easier to standardize as a method.

It does not indicate the sizes of the particle. Remember the example of a ball bearing in a sample: a solid ball bearing and the same one ground to powder should give the same PQI.

Used in conjunction with the ICP iron reading, the PQI is invaluable in estimating the distribution of wear particle sizes. Table 4 shows this relationship. High, medium and low are relative concepts and should be interpreted in the context of other samples in the component's history.

Situation	Icp Iron (Fe)  [ppm]	PQI	Inference	Wear profile
1	Low	Low	Few wear particles	Normal wear profile
2	High	Low to medium	Lots of small particles, few or no large onews	Accelerated wear (type of operation) Wet-brake systems (normal or abnormal) Dirt entry (abnormal)
3	Low	High	Few small particles, many large ones	Fatigue
4	High	High	Lots of particles of all different sizes	Serious wear likely, catastrophic failure possible

Situation 2 has various possible origins. It can be typical of a component experiencing accelerated but not abnormal wear; that is, the component is working harder than normal. This is illustrated by comparing the wear readings of differentials of identical trucks in different operations, for example, short and long-haul operations.

Differences in what can be considered normal wear for each situation can be up to two orders of magnitude. This situation is also typical of normal brake wear in immersed-brake systems (such as most front-end loaders). Dirt entry causing abnormal wear also generates this Fe-PQI relationship.

The LaserNet Fines Reports large Ferrous concentration (PPM), percentage of large Ferrous particles (PLFP), Ferrous Wear Severity Index (FWSI).

The capability of the LaserNet Fines to capture the actual wear particle silhouette allows for an ‘Automated Ferrography’ capability for wear particle classification. All particles larger than 20 μ are classified by a neural network in categories such as cutting, fatigue, severe sliding, non-metallic, free water and fibers.

Viscosity

There are two types of viscosity: kinematic and dynamic (or absolute). Oil analysis concerns itself almost exclusively with the former. Kinematic viscosity is measured in centistokes (cSt) and is a measure of a fluid’s resistance to flow or, more simply, its thickness. It must always be quoted at a stated temperature because a fluid’s viscosity will change with temperature. At 40°C, a 200 cSt oil is thicker than a 100 cSt one.

Most labs carry out a viscosity measurement at 40°C on every sample. A viscosity measurement at 100°C can also be carried out on machines which operate at high temperatures, such as engines and some compressors.

The process is simple: a glass tube (the ends of which are kept open to the air) is immersed vertically in a bath at the required temperature; oil is introduced at the top and, as it flows down, it is brought up to the correct temperature. Its flow is then timed between two marks. The time measurement is converted to a viscosity.

There is another property of an oil related to its viscosity. This is the viscosity index (VI). It is known that as the temperature of an oil increases, its viscosity decreases. The VI of an oil indicates how much it is going to thin out.

A monograde oil has a lower VI than a multigrade, meaning the monograde tends to thin out more than the multigrade with increasing temperature. For example, a typical SAE 30 monograde and a typical SAE 15W40 multigrade can both have a viscosity at 40°C of 100 cSt. But at 100°C they have respective viscosities of 10 and 15.

To determine the VI of an oil, measure its viscosity at both 40°C and 100°C.

Table 5 shows some of the causes of changes in viscosity. It is important to note that concurrent conditions can mask the effects of changes of viscosity. Fuel dilution accompanied by overheating could leave the viscosity reading looking normal.

Component	Viscosity change	Cause
Engine	Increase	Overheating (may or may not be accompanied by oxidation)
		Sludging (poor combustion or overextended use)
		Fuel dilution (marine engines fired with heavy fuel oil)
		Severe water contamination
	Decrease	Fuel dilution
		Breakdown of VI improver additive in multigrade oils with extended use
		Overheating
Other components	Increase	Grease contamination
		Severe water contamination
		General breakdown of the oil
		Mixture of oils
	Decrease	Contamination by a volatile substance
		Breakdown of VI improver additive (particularly noticeable in transmissions filled with a multigrade)
		General breakdown of the oil

Table 5. Changes in Viscosity

Once again, the importance of submitting accurate information should be emphasized. Perfectly good oil may be recommended for change-out due to discrepancies between the oil grade in the machine and the oil grade identified in the paperwork.

Furthermore, an engine described as having an SAE 30 or a SAE 15W40, but actually running an SAE 40 or SAE 20W50, may go unchecked for fuel dilution, because the decreased viscosity resulting from fuel dilution may compare favorably with the normal viscosity of the described oil.

The MiniVisc 3000 series viscometers are the first truly portable, solvent-free, temperature-controlled kinematic viscometers, providing high-accuracy measurements for easy detection of viscosity variations caused by contamination, mix-up and oil degradation. 

Water

Water is one of the more common contaminants. If it can be introduced via internal coolant leaks, high-pressure hose cleaning procedures or condensation. Water has several negative effects on the performance of oil, including:

• Formation of rust, which in turn contaminates the oil.
• Increased wear rate from decreased lost load-bearing capacity.
• Creation of weak and strong acids from chemical reactions between additives and base oils.
• Biological formation and growth in low-temperature applications.
• Loss of critical additives and additive function.

It is important that water contamination be kept to the absolute minimum. Seals and breathers should be regularly inspected and maintained. Pressurized cooling systems need to be pressure-tested regularly to confirm their integrity.

Engine samples are screened for water using Fourier transform infrared (FTIR) analysis and every other sample is screened for water using the crackle test. This test involves putting a drop of oil onto a steel surface which is maintained between the boiling points of water and oil.

If the oil drop contains water it spits and crackles, hence its name. The crackle test can detect water contamination of less than 0.1 percent, or 1,000 ppm. If a sample fails the crackle test, the actual water content is measured. Once again, tentative limits for water contamination are used (Table 6), although these will vary in situations of abnormal or unusual usage.

Component	Limit [%]
Engine	0.0
Drivetrain	1.0
Transmission	0.5
Hydraulics	0.5
Compressors	Variable according to type

Table 6. Water Limits

Water should not be relied upon as an indication of an internal coolant leak, particularly in engines. It tends to evaporate at normal operating temperatures.

The FluidScan® 1000 series handheld Infrared oil analyzer provides a direct quantitative measurement of water, TAN and oxidation for lubricants used in gearboxes, turbines and hydraulic systems. 

Bottom Line

Together, these four tests provide a strong foundation for understanding lubricant condition and machine health. Running them consistently ensures that wear, contamination, and oil degradation are identified before they escalate into costly failures. When oil analysis can be performed on site, the benefits are amplified by faster turnaround, immediate context from the equipment, and quicker decision-making. Instead of waiting days for results, maintenance teams gain timely insights that support corrective action while the machine is still operating, turning oil analysis from a diagnostic exercise into a proactive reliability tool.

Read more on oil analysis best practices:

How to Select the Right Oil Analysis Lab

Statistical Techniques to Simplify Oil Analysis Data

How to Interpret Oil Analysis Reports

Editor’s Note: 

This article was authored by Ashley Mayer but was originally published as Technical Bulletin Issue 19 by Wearcheck Africa, a member of the Set Point group.