Multigrade vs Monograde Oil: Understanding Engine Protection Through Viscosity

Motor oil plays a critical role in engine health, with its primary function being to reduce friction between moving parts. It achieves this by forming a protective film that separates mating surfaces. When this film is inadequate, metal components may come into contact, leading to increased wear and potentially irreversible damage. One of the key indicators of an oil’s ability to form this protective layer is its viscosity.

What Is Viscosity?

Viscosity refers to a liquid’s resistance to flow. Put simply, it’s a measure of a fluid’s “thickness”. Water flows much more easily than honey, for instance, because it has a lower viscosity. From an engineering perspective, a high-viscosity oil is generally more effective at separating surfaces. However, oil also needs to be fluid enough to circulate easily throughout the engine. Thus, the ideal oil strikes a balance between being thick enough to protect, yet thin enough to flow efficiently.

Temperature adds complexity to this equation. As oil heats up, its viscosity decreases; conversely, it thickens when cold. That’s why viscosity is always measured at specific temperatures—typically at 40°C and 100°C for lubricating oils. Since modern engines typically operate around 100°C, the viscosity at this temperature is very important, as shown in the last two columns in the table below:

SAE J300 and Monograde Oils

Engine oils are classified under the SAE J300 viscosity grading system. Monograde oils are those that meet a single viscosity requirement—either for low or high temperature. In this system, the higher the SAE number, the thicker the oil. Low-viscosity oils suitable for winter use carry a “W” (e.g. SAE 15W), and they must meet certain cold temperature performance standards to ensure effective protection during engine start-up in colder climates.

If we draw graphs of a typical SAE 15W and SAE 40 monograde oil with viscosity (plotted as a logarithmic function) on the vertical axis against temperature (as a linear function) on the horizontal axis, we come up with the blue and red lines in the diagram below:

• SAE 15W flows well in cold conditions, offering protection during cold starts, but becomes too thin at higher operating temperatures.

• SAE 40, on the other hand, performs effectively at operating temperatures but is too thick to flow quickly during cold starts.

This highlights a problem: monograde oils cannot provide optimal protection across the full range of engine temperatures.

The Multigrade Solution

The answer lies in multigrade oils, which are engineered to behave like two different oils depending on the temperature. This is achieved by blending a base oil with a viscosity modifier (VM), also known as a viscosity index improver.

Viscosity modifiers are smart additives that respond to temperature changes. At low temperatures, they contract and do not significantly alter the oil’s viscosity. As the temperature rises, they expand, increasing the oil’s viscosity and helping it maintain its protective film.

For example, starting with a base oil equivalent to SAE 15W, and adding the right amount of VM to achieve SAE 40 performance at 100°C, results in a SAE 15W-40 multigrade oil. This oil flows well at low temperatures and maintains appropriate thickness at high temperatures. Other common multigrades include SAE 5W-30, SAE 10W-40, and SAE 20W-50.

Why Multigrade Oils Are Preferable

Multigrade oils offer superior engine protection across a wider range of temperatures compared to monogrades. They ensure:

• Easier starting and quicker oil circulation in cold weather
• Sustained film strength and wear protection at normal operating temperatures

This dual performance makes multigrade oils ideal for modern engines that experience both high and low temperature extremes during regular operation.

Need Expert Advice?

If you have any questions about engine oils or lubricants in general, feel free to get in touch with our team at info@bcl.co.za. Our lubrication specialists are ready to help with expert advice and support tailored to your needs.