Understanding Motor Oil Viscosity

When speaking of the properties of a motor oil, viscosity is of tremendous importance. Unfortunately, it also an area that is very confusing. The viscosity of an oil refers to its relative resistance to flow at differing temperatures. As has been previously mentioned, an oil must not only be "thin" enough to flow well at low temperatures, it must also remain "thick" enough to maintain adequate protection at high temperatures.

Ideally, an oil will maintain a consistent viscosity over a wide temperature range. The viscosity index of an oil is a measure of its tendency to change viscosity with temperature changes. The higher the viscosity index (VI) the more consistent an oil's viscosity is with temperature changes.

Monograde Oils

Monograde oils such as 30 weight oils are designed for consistent temperature applications. For instance, you will find that most older lawn tractors and mowers call for a straight 30 weight oil (SAE 30). This is because it is assumed that these will be operating mainly in warm temperature summer months.

So, if you take a look at their viscosity index, you'll notice that most monograde oils have a low VI number. This implies that as you cool the oil it will thicken quite a bit. However, this is ok, because the oil is designed to only be used under warm conditions. Cold temperature thickening will not be an issue.

According to SAE J300 standards, to be classified as a certain SAE viscosity, an oil is heated to 100 degrees C (212 degrees F). It's kinematic viscosity at this temperature is measured. If it falls within a certain range it is classified as a particular viscosity. For instance, an SAE 30 oil must have a kinematic viscosity at 100 degrees C of between 9.3 and 12.5 cSt (centistokes).

Multi-Viscosity (Multi-Grade) Oils

Multi-viscosity (multi-grade) oils such as 0w30, 5w30, 10w40 and so on are oils, which are designed for applications where temperature changes may be significant. For instance, multi-viscosity oils might be used in northern US climates where temperatures can be -20 degrees F in the winter and +95 degrees F in the summer.

However, that does not mean they cannot be used for applications where the temperature remains more consistent. The fact is, monograde oils are becoming much less common as multi-viscosity oils are being substituted in applications which traditionally called for a monograde oil.

Nevertheless, monograde oils are still used in many super high performance racing applications, construction equipment that is used in only summer months and industrial engines that are kept indoors at a constant temperature all year round.

Additional Info via the Links Below:

page 1 - current : Intro - Mono vs. Multi-Grade
page 2 : What Do the Numbers Mean?
page 3 : The Problem With Multi-Viscosity Oils
page 4 : The Solution For Multi-Viscosity Oils