Oil Parameters in Lubrication Management and Factors Affecting Their Changes

In industrial sites where a wide variety of machinery and heavy equipment operate, lubricants such as gear oil, hydraulic oil, and engine oil are indispensable. Each lubrication point requires an oil grade specifically suited to its unique operating conditions, including load, speed, and environment. Therefore, the first step in maximizing machine performance is selecting the right oil that matches the machine type and its operating conditions.

However, oil gradually deteriorates as the equipment operates. Even if the appropriate oil is used initially, it will lose its original properties over time. This makes timely oil replacement essential. But how exactly does oil degradation occur? And how can we determine the extent of that degradation?

At many sites, the timing of oil changes is determined by operating hours or mileage. Yet, by performing oil analysis, it is possible to determine a more accurate and appropriate timing for replacement.

In this article series, divided into Part 1 and Part 2, we will explore in detail:

• What are the key oil parameters involved in lubrication?
• Why do oil properties change as machines operate?
• What kinds of analytical methods are effective in assessing oil degradation?

We will answer these questions as thoroughly as possible to help you better understand lubrication management and the role of oil analysis in maintaining machine reliability.

Oil Parameters in Lubrication Management

Viscosity

When selecting oil, the most important point of focus is undoubtedly viscosity. The viscosity grade, often referred to as the “oil number,” indicates how thick or thin the oil is. For example, when we say “No.150 oil,” it means the oil has an ISO viscosity grade of 150.

Gear oils, hydraulic oils, and engine oils are all classified according to their viscosity. The machine’s specification sheet typically lists the required viscosity grade for each lubrication point.

So, what exactly happens when the viscosity grade differs? How does the viscosity of an oil affect its lubricating performance?

As the term suggests, oil viscosity refers to the “thickness” or resistance to flow of an oil. The higher the viscosity number, the thicker and more viscous the oil; the lower the number, the thinner and more fluid it is.

Viscosity is typically measured in centistokes (cSt), though square millimeters per second (mm²/s) is also sometimes used. Strictly speaking, these values represent kinematic viscosity, not “dynamic viscosity.” However, in practical field use, the term “viscosity” is commonly used to describe both kinematic viscosity and dynamic viscosity.

Although there are two different units, they are equivalent — 1 cSt = 1 mm²/s — so they can be used interchangeably without issue.

There are also two standard viscosity grading systems: ISO viscosity grades and SAE viscosity grades.

• ISO viscosity grades are primarily used for industrial oils, including gear oils and hydraulic oils.
• The ISO viscosity grade represents the oil’s viscosity measured at 40°C.

For example, an ISO VG 150 oil means the oil has a viscosity of approximately 150 cSt at 40°C.

The reason viscosity grades are defined “at 40°C” is that an oil’s viscosity changes significantly with temperature.

For example, an ISO VG 220 gear oil will have a viscosity of approximately 220 cSt at 40°C, but when measured at 100°C, its viscosity drops to around 20 cSt.

Since oil tends to lose viscosity rapidly as temperature increases, it is essential to specify not only the viscosity value but also the temperature at which the measurement was taken.

Incidentally, the fact that oil viscosity decreases as temperature rises is something we can easily observe in everyday life.

For example, when cooking stir-fry, the vegetable oil you pour into a frying pan becomes thinner and flows more easily as the pan heats up, spreading smoothly across the surface. This happens because as the oil’s temperature increases, its viscosity decreases — a simple, familiar demonstration of the same principle.

To return to the main topic, the reason oil viscosity is so important can be summarized in one phrase: it’s all about maintaining the balance between load-carrying capacity and viscous resistance.

Generally, high-viscosity (thicker) oils offer better film strength under heavy loads, ensuring that metal surfaces remain properly separated. However, such oils also create greater internal resistance during operation, which can lead to higher temperatures and increased energy consumption.

In high-speed equipment, this resistance becomes more pronounced, making the use of overly viscous oils risky. Therefore, as a general rule:

• High-load, low-speed machinery uses high-viscosity oils,
• while low-load, high-speed machinery requires low-viscosity oils.

Suppose the oil’s viscosity changes due to degradation. In that case, it can no longer maintain the proper lubricating film thickness, or it may cause excessive frictional heat—both of which can lead to serious mechanical problems.

Additives

　While the main component of oils sold by various manufacturers is, of course, the base oil—such as mineral oil or synthetic oil—they also contain a variety of additives that enhance performance and protection.

Examples of such additives include:

• Extreme Pressure (EP) Additives, which prevent seizure and wear when localized high pressure occurs;
• Viscosity Index Improvers, which help stabilize viscosity across temperature changes;
• Emulsion Inhibitors, which prevent the formation of emulsions in oil-water mixtures.

Oils that contain a wide range and high concentration of additives tend to be more expensive, but they also deliver superior performance and protection, especially under demanding operating conditions.

　The challenging aspect of additives is that it’s difficult for users to determine which additive components and in what quantities are necessary for optimal performance. To address this, oils are classified by performance grades that are defined separately from viscosity.

For example, in diesel engine oils, grades such as CF-4 and CK-4 are used. Each grade indicates that the oil meets specific standards and performance requirements, including those related to the type and quality of additives used.

In this grading system, the series begins with CA, followed by CB, CC, CD, CE, CF-4, CI-4, CK-4, and so on. As the letter following “C” progresses through the alphabet, the performance criteria become more stringent.

Therefore, if an engine specification calls for CF-4 oil, it is generally acceptable—and often preferable—to use any oil from CF-4 up to CK-4, since higher grades meet or exceed the requirements of the lower ones.

As explained above, by checking the viscosity and grade markings, you can select the oil that best suits your machinery or equipment. However, just like passenger cars require regular oil changes, the performance of industrial oils also deteriorates over time as the equipment operates. So, how exactly does this degradation process occur? What mechanisms cause the oil’s properties to change during use?

Why Do Oil Properties Change During Machine Operation?

Shear-Induced Degradation

　The term “shear” may sound unfamiliar, but it is actually one of the most common causes of oil degradation. The word “shear” refers to a force that acts when two parallel surfaces move in opposite directions, sliding against each other.

While this may sound technical, it’s easier to understand if we imagine what happens between gears and oil. On a microscopic level, the tooth surfaces of two gears move in opposite directions as they rotate. The oil film, along with its additives, forms a thin layer between these surfaces and is continuously pulled in opposite directions by the motion of the gears.

When this shear stress is applied repeatedly, the molecular structure of the oil and its additives breaks down over time, resulting in a decrease in viscosity in most cases.

In other words, shear degradation occurs as the oil film is stretched and torn by opposing mechanical forces during operation — a natural but critical process that affects the oil’s ability to lubricate and protect machinery.

Oil Oxidation

Oxidation is a common issue that affects nearly all types of oils. It progresses through a chemical reaction between the oil’s components and oxidizing substances such as oxygen. Under normal storage conditions, oxidation advances very slowly. Therefore, while it’s not ideal, even if the lid of an oil pail is accidentally left open for several months in a warehouse, the oil will not usually become fully oxidized. In other words, oxidation during storage is minimal compared to the rate of oxidation that occurs when the oil is actually in operation under heat and pressure.

The problem lies in the fact that chemical reactions are accelerated by heat. Generally, it is stated that for every 10°C increase in temperature, the rate of a chemical reaction doubles. For example, compared to oil at 80°C, oil at 90°C oxidizes twice as fast, at 100°C it oxidizes four times as fast, and at 110°C it oxidizes eight times as fast. Therefore, oils that operate at high temperatures are particularly susceptible to oxidation. When oil oxidizes, its viscosity changes, which affects its ability to form a proper lubricating film. As oxidation progresses further, varnish and sludge begin to form, which can lead to increased wear and damage to components within the equipment.

Incidentally, oil oxidation is a phenomenon we can easily observe in everyday life. For example, vegetable oil retains its light yellow color for quite some time after opening the bottle, but after being used for deep-frying, it turns brownish and its flavor changes. This happens because heating accelerates the oxidation of the oil.

Emulsification

　Although emulsification is not as common as oxidation, it can significantly alter the performance of lubricating oil. Emulsification is a phenomenon in which oil and water undergo a chemical reaction, making it increasingly difficult to separate the oil from the water. As this process advances, the oil takes on a milky, milkshake-like appearance. In closed systems such as hydraulic oils, emulsification rarely occurs. However, in outdoor equipment—for example, gearboxes exposed to the environment—it is quite a common issue. Like oxidation, emulsification is accelerated by heat, meaning that high ambient temperatures in summer, as well as heat and friction generated during operation, can greatly increase the rate of the reaction. When oil becomes emulsified, not only does its viscosity change, but it also loses its ability to separate from water, leading to direct contact between water and gear surfaces. This greatly increases the risk of corrosion, making emulsification a deceptively serious problem that should not be underestimated.

Additive Depletion

As mentioned earlier, various additives are blended into oils to enhance their performance, but some of these additives are gradually consumed as the machinery operates. For example, organic molybdenum, a commonly used extreme-pressure (EP) additive, functions by undergoing a chemical reaction triggered by the localized heat generated under high pressure. Through this reaction, the organic molybdenum compound transforms into another substance, resulting in a decrease in its concentration in the oil over time as the equipment continues to operate.

Similarly, engine oils contain alkaline additives designed to neutralize acids produced during fuel combustion. These alkaline components are also gradually depleted during operation.

Unlike oxidation or emulsification, which can sometimes be detected through changes in odor or appearance, the extent of additive depletion cannot be perceived by human senses. Therefore, even if the oil shows no noticeable visual change at the time of replacement, its performance may have already declined due to the loss of active additives.

Contamination

　Unlike the previously discussed issues, contamination by foreign substances does not technically constitute a deterioration of the oil itself; however, it remains a critical factor in lubrication management and must be addressed. Contamination can generally be divided into two main categories.

The first case involves contaminants entering the lubrication system from outside sources. In older equipment, corrosion or seal deterioration may allow dust, sand, or dirt to enter from the external environment. Even in newer equipment, such contaminants can be introduced during oil changes or maintenance procedures.

These foreign particles are typically more complex than metal surfaces, so when they become trapped between gear teeth, they can cause significant damage to the metal components, leading to abrasion, pitting, or surface wear.

The second case involves contaminants generated within the lubrication system itself. This typically occurs when metal wear particles are produced under extreme pressure conditions, or when carbon residues form as a result of oil oxidation.

This issue can arise even in new equipment and despite careful oil replacement practices, making it a particularly troublesome problem. Moreover, it is often accelerated by other degradation processes such as oxidation and emulsification, which further contribute to internal contamination.

Among the contaminants generated within the lubrication system, particles larger than approximately 4 micrometers (µm) are considered harmful to lubrication performance. For reference, the average thickness of a human hair is about 80 µm, which helps illustrate how extremely small — and nearly invisible — these harmful particles are.

This means that even if the oil drained from a machine during an oil change appears clear and clean to the naked eye, it may still contain a large amount of microscopic contaminants capable of causing wear and reducing the effectiveness of lubrication.

In this article, we introduced the key oil parameters involved in lubrication management and discussed in detail the various problems that can occur in oils as machinery operates.

In the next article, we will explain how oil analysis can be used to detect, manage, and ultimately resolve these issues, helping ensure optimal equipment performance and reliability.