Viscosity is a measurement of an oil’s ability to flow. Oils can range from viscosities as thin as water to viscosities as thick as asphalt, so there is quite a large window of possibilities to work with when formulating lubricants. There are two varieties of viscosity in which to measure oil. The first is the kinematic viscosity which is measured by the flowing characteristics of oil. The second is called the dynamic viscosity and is measured by the resistance oil exerts on an object pushing through it. Although dynamic viscosity is used to measure properties in motorcycle lubricants, the kinematic viscosity is the biggest factor in classifying oils for various motorcycle applications.
Kinematic viscosity is the main measurement used to differentiate the Society of Automotive Engineers (SAE) various grades of engine oils and gear oils. This is essentially a measurement of the “thickness” of oil and specific ranges of viscosity correspond to specific SAE viscosity grades. At the time of this writing, the SAE currently has six active viscosity grades (60, 50, 40, 30, 20 and 16) with another two (12 and 8) being set for future implementation.
In addition to the kinematic viscosity requirements for an SAE grade, there is an accompanying high temperature high shear (HTHS) viscosity measurement to go along with each grade. Although its name would suggest otherwise, this is not a measurement of the shearing protection an oil provides but instead the viscosity measured under different conditions than the normal kinematic viscosity that can be likened to how the oil will slide over top of itself as opposed to how it flows.
One last point of the SAE viscosity grades is a pair of low temperature viscosity requirements which are related to the “W” grades or the winter grade of an oil (the W stands for winter, not weight as commonly believed). These two measurements indicate the oil’s ability to be pumped through an orifice (its flow) and its ability to let an object pass through it (its solidity).
Table 1: SAE J300 Engine Oil Viscosity Specification
Table 1 illustrates the differences between each possible grade of engine oil defined by the SAE. Multigrade oils come with a few rules. The winter grade comes first (for example: 5W-40, 10W-30, 20W-50), any oil that has a multigrade designation needs to meet all of the requirements of both grades indicated, and oil marketers are required to print the highest performance grade that the oil meets on the container of oil. This means that oil companies cannot produce a 10W-50 grade oil and market it as a 20W-50 even though it would technically meet and exceed all of the specifications of a 20W-50 because 10W is higher performance than 20W
Gear and transmission oils follow a similar set of requirements with required ranges of the kinematic viscosity at 100°C and a low temperature viscosity requirement.
Table 2: SAE J306 Gear & Transmission Oil Viscosity Specification
The two tables presented above are the one and only standard way to classify engine and gear oils in the motorcycle lubricant industry. There are other ways to categorize products for use in industrial applications, but they have no place in these discussions. Oils can either be mono-grade meaning they only claim to meet one viscosity specification (ie. 80W transmission oil and 50 weight engine oil) or they can be multi-grade meaning they meet the specifications for two of the classifications (ie. 5W-40 and 25W-60). The specifications are created so that only one winter grade and one weight can be claimed at any point.
Many believe that the winter grade designates how thick an oil will be, as in thinking a 10W-50 will be thinner than a 20W-50 but this is not true. Remember, the winter grade only correlates to an oils behavior at subzero temperatures. The fact that the two oils are 50 weights says that they need to meet the same viscosity range at high temperature. That being said, the tendency for an oil to have a lower viscosity at low temperature (which is desirable) does tend to also make it seem thinner at room temperature so this is where this myth probably originates, but most people see the oil at ambient temperature as they pour it out of a bottle and not as it pumps through a motor at 100°C so it is an understandable belief. However, this is the fact to take away, any 50 weight engine oil (or 20, 30, 40 or 60) behaves exactly like any other 50 weight engine oil (or 20, 30, 40 or 60 respectively) at the operating temperature of an engine. The lower the “W” grade of an oil, the more thermal stability it generally has which is a desirable property in oils, but it is not a given so I don't recommend assuming one oil is better than another just based on that property.
Viscosity is the most basic property of oil and it is without a doubt the most important feature of any oil. Using the appropriate viscosity oil is the first correct choice to make in selecting any lubricant for use in your machines. The machine manufacturers designed the engine or gearbox to operate with a specific lubricant so things like the oil channels, oil pump and clearances are all specific to the recommended grade of oil. The one and only exception to this is the winter grade. You can always go to a lower “W” grade as long as you maintain the regular grade (ie. you can use a 10W-50 instead of a recommended 20W-50). You can sometimes go to a higher “W” grade as well, but that depends on the application and environment and is only advised when the climate does not approach freezing temperatures.