LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

LO 10-02: Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils.

Oil Temperature and Viscosity

When it is hot, oil thins and becomes less viscous. Likewise, oil thickens and the viscosity increases as the temperature decreases Figure 15. When an oil’s kinematic viscosity is measured at 0° F (−18° C), it is given the suffix W, indicating that it has a winter viscosity rating. This scale for measuring viscosity is also known as the absolute viscosity rating . Oil is also tested at 212° F (100° C), which is the expected operating range of oil inside an engine. When measured at this temperature, the SAE kinematic viscosity is referred to as a hot or summer viscosity and no letter accompanies the SAE viscosity grade.

Multigrade Oils
Oils with only a single viscosity number are designated single-grade or single-weight oils. Single-grade oils are obsolete for use in any modern diesel because of the introduction of multigrade or multi-weight oils. Multigrade oils can be a blend of several different oils with different viscosities or oils containing viscosity- changing additives Figure 16. Multigrade oil viscosity today is indicated by two numbers (e.g., 15W-40), which means they have their viscosity measured under both hot and cold conditions. The numbers provide viscosity information about the oil’s performance under a wide range of operating conditions.

The first number (e.g., 15W in 15W-40) is the low-temperature viscosity of the base oil, and it helps predict how well an engine might crank in cold temperatures when the oil is thick and resistance to cranking is high. Generally, a lower number means the oil is thinner and less resistant to flowing between moving parts, allowing the engine to start easier in cold weather. The low-temperature or winter number also indicates how well the oil will flow to lubricate critical engine parts during severe cold-starting conditions Figure 17. Colder temperatures require oil with a lower or thinner viscosity to properly lubricate an engine. Pour point depressants—additives that lower the oil’s cold temperature viscosity—may be added to give the oil this property. The use of an engine block and an oil pan heater can also help warm oil to prevent it from thickening too much when the engine is shut down. The second number (e.g., 40 in 15W-40) is the high-temperature viscosity of the oil when it contains an additive to slow down the thinning of the oil’s viscosity as its temperature increases. The danger of hot oil is that its viscosity becomes too low, and it cannot develop a proper hydrodynamic oil wedge. The use of additives to thicken hot oil or mixing oils with different viscosities prevents the oil from becoming too thin and unable to protect the engine. The oil’s second viscosity number does not have a W suffix and is measured at 212° F (100° C).

FIGURE 10-15: Temperature changes an oil’s viscosity. Multigrade oils have a lower viscosity index than single-weight oils. This means multigrade oil viscosity does not change as dramatically with temperature.

FIGURE 10-16: Temperature changes an oil’s viscosity. Additives and blends of oil with different viscosities can alter the rate at which oils change viscosity with temperature.

FIGURE 10-16: Selection of oil viscosity for a diesel engine is dependent upon the operating temperatures expected. Most heavy-duty engines in North America built before 2010 operated using SAE 15W-40-grade engine oil.

©2026 - CDX