Limit and warning levels from elemental spectrometric analysis serve as indicators of the amount of foreign particles found in used oil that is still tolerable or, when compared with fresh oil, indicate when the altered lubricant must be changed. Values well above tolerable wear levels can also indicate an acute damage process. However, it is not easy to specify these warning levels. Hardly any engine or equipment manufacturer defines limit levels for used oil. This is because the operating conditions and times are too specific, and the origins of the foreign particles found in the oil are too diverse. Consequently, determining these factors is one of the essential tasks of every oil analysis. After all, the type, quantity and (to a certain extent) the size of the particles provide valuable information about wear, contaminants and the additives in the oil.

When warning and limit levels are used for the diagnosis of a specific oil specimen, the interactions between the values and other criteria should also be taken into account. A variety of factors play a role here, including the engine manufacturer, the engine type, the type of fuel used, the oil volume, the motor oil type, the service life of the motor oil, and any top-up quantities (makeup oil). The operating conditions can also vary markedly from one situation to the next. After all, the engine of a heavy construction machine operates under different conditions than the same engine of a truck traveling long distances on a highway at uniform speed.

However, all of these engines have one thing in common: their motor oil contains a lot of valuable information about the oil itself as well as the state of the engine. For example, the microscopic particles suspended in the oil provide an indication of the amount of wear of the corresponding parts or components. Elements such as sodium, potassium or silicon indicate contamination by road salt, hard water, glycol antifreeze or dust. Comparing the amount of organometallic additive elements (such as calcium, magnesium, phosphorus, zinc, sulfur or boron) in the used oil to fresh oil provides an indication of changes to the oil, such as additive depletion or possibly the mixing of different types of oils.

Table 1. Wear elements

Inductively coupled plasma (ICP) elemental analysis can be used to determine more than 30 different elements in motor oils. In addition to the presence of the elements, atomic emission spectroscopy (AES) by ICP can be used to determine the concentrations of the elements.

Laboratories routinely determine the following elements and values as part of motor oil testing and list them in the lab report: iron, chromium, tin, aluminum, nickel, copper, lead, calcium, magnesium, boron, zinc, phosphorus, barium, molybdenum, sulfur, silicon, sodium and potassium. In some cases, other elements are also determined, such as silver, vanadium, tungsten or ceramic elements like cerium and beryllium, which are rarely present in motor oils. They are only listed in the lab report if they are actually proven to be present or if the customer specifically requests this. Tables 1-3 show the possible causes for the presence of the elements found in oil, i.e., whether they are related to contaminants, wear or additives.

Table 2. Contaminants

Various factors must be taken into account when interpreting a lab report and the values of the elements found in the oil. Naturally, it is not sufficient to simply report the elements and their quantities. In order to assess the measured values, you must know whether the individual elements indicate contamination, wear or changes to the additives. However, these values are also interrelated to a certain extent. The relative proportion of various wear elements provides an indication of the affected machine parts or components, for example. Further, it is important to know how long it has taken for the oil to become enriched with specific wear elements since the last oil change. The operating time of the overall system or the running time of the engine, the oil volume relative to the engine power, and the top-up amounts must also be considered when analyzing or diagnosing warning levels.

In order to reliably assess the values determined for the used oil and their relationship to each other and to other factors, it is necessary to have a suitably large volume of data and analytical expertise. However, additive elements and base oil types can differ considerably depending on the type of oil used, so it is necessary to set suitably broad warning levels. Specific warning levels can only be defined for a specific oil type.

Table 3. Additives

The warning and limit levels listed in Tables 1-3 for wear elements, contaminants and additives are based on a semi-synthetic motor oil (SAE 10W-40) in a modern diesel engine with an oil volume of approximately 25 to 50 liters, using fuel compliant with EN 590 (containing 5 percent fatty acid methyl esters), and with an oil service life of approximately 500 operating hours or a mileage of approximately 47,000 miles.

The basic rule is that warning levels must be set lower for greater oil volume, shorter oil service life, lower engine speed and lighter load conditions.

However, the stated values are distinctly dependent on the oil manufacturer, the correct engine type, the service life of the oil charge, the oil volume and the top-up quantities (if any).