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Oil viscosity is a measurement of oil thickness. Thick oil provides a heavier barrier between moving parts. However, thick oil cannot be easily distributed in an engine that is cold--and that is when most engine damage occurs. Simply put, viscosity is the oil's resistance to flow or, for the layman, an oil's speed of flow as measured through a device known as a viscometer. The thicker (higher viscosity) of an oil, the slower it will flow. You will see oil viscosity measurement in lube articles stated in kinematic (kv) and absolute (cSt) terms. These are translated into the easier to understand SAE viscosity numbers you see on an oil bottle.
Temperature
Cold oil is thick, and as it is heated it becomes thinner. When a cold engine is started, thick oil cannot flow into smaller openings. This is called "dry running," and according to the UpMPG website, "Less dry running means much less engine wear."
Viscometer
To measure viscosity, oil is heated to 100 degrees Celsius, which is considered the approximate temperature of a warm engine. The oil is then placed in a measuring device called a viscometer.
Time
The length of time it takes the oil to flow out of the viscometer determines its viscosity. Thicker oil will take longer to flow out of the viscometer, and thus it will have a higher viscosity rating.
Viscosity vs. Weight
Many people refer to the thickness of oil as "weight"--such as "40-weight" oil. Weight and viscosity mean the same thing and may be used interchangeably.
Multi-weight
Modern oils may have a designation of a number, letter and number, such as 10W40. This example is a multi-weight (viscosity) oil. That is, it will perform as thinner (10 weight) oil when your engine is cold and as a thicker (40 weight) oil when your engine warms, thereby protecting your engine when cold and warm.
What does a 5W-30 do that an SAE 30 won't?
When you see a W on a viscosity rating it means that this oil viscosity has been tested at a Colder temperature. The numbers without the W are all tested at 210° F or 100° C which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210° (100° C). The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F (100° C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.
Obviously, cold temperature or W ratings are tested differently than regular SAE viscosity ratings. Simply put, these tests are done with a different temperature system. There is a scale for the W, or winter viscosity grades and, depending on which grade is selected, testing is done at different temperatures. See the Tables to the right below for more information.
If you look at the table, SAE Viscosity Chart (High Temp) you'll see that if a measured amount of motor oil flows through the viscometer at 210° F (100° C) faster than 5.6 but less than 9.3 seconds, then it will be considered a SAE 20 viscosity. Consequently, if a motor oil flows through faster than 9.3 and slower than 12.5 seconds, then it will be a SAE 30 viscosity.
Now if you look at the table labeled Winter or "W" Grades, you can get valuable information on how the W or winter grade viscosities are measured. Basically, as shown by the chart, when the oil is reduced to a colder temperature it is measured for performance factors. If it performs like a SAE 0 motor oil at the colder temperature, then it will receive the SAE 0W viscosity grade. Consequently, if the motor oil performs like a SAE 20 motor oil at the reduced temperatures (the scale varies - see the chart), then it will be a SAE 20W motor oil.
If a motor oil passes the cold temperature or W (winter grade) specification for a SAE 15W and at 210° F (100° C) flows through the viscometer like a SAE 40 motor oil, then the label will read 15W-40. Getting the picture? Consequently, if the motor oil performs like a SAE 5 motor oil on the reduced temperature scale and flows like a SAE 20 at 210° F (100° C), then this motor oil's label will read 5W-20. And so forth and so on!
Temperature
Cold oil is thick, and as it is heated it becomes thinner. When a cold engine is started, thick oil cannot flow into smaller openings. This is called "dry running," and according to the UpMPG website, "Less dry running means much less engine wear."
Viscometer
To measure viscosity, oil is heated to 100 degrees Celsius, which is considered the approximate temperature of a warm engine. The oil is then placed in a measuring device called a viscometer.
Time
The length of time it takes the oil to flow out of the viscometer determines its viscosity. Thicker oil will take longer to flow out of the viscometer, and thus it will have a higher viscosity rating.
Viscosity vs. Weight
Many people refer to the thickness of oil as "weight"--such as "40-weight" oil. Weight and viscosity mean the same thing and may be used interchangeably.
Multi-weight
Modern oils may have a designation of a number, letter and number, such as 10W40. This example is a multi-weight (viscosity) oil. That is, it will perform as thinner (10 weight) oil when your engine is cold and as a thicker (40 weight) oil when your engine warms, thereby protecting your engine when cold and warm.
What does a 5W-30 do that an SAE 30 won't?
When you see a W on a viscosity rating it means that this oil viscosity has been tested at a Colder temperature. The numbers without the W are all tested at 210° F or 100° C which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210° (100° C). The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F (100° C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.
Obviously, cold temperature or W ratings are tested differently than regular SAE viscosity ratings. Simply put, these tests are done with a different temperature system. There is a scale for the W, or winter viscosity grades and, depending on which grade is selected, testing is done at different temperatures. See the Tables to the right below for more information.
If you look at the table, SAE Viscosity Chart (High Temp) you'll see that if a measured amount of motor oil flows through the viscometer at 210° F (100° C) faster than 5.6 but less than 9.3 seconds, then it will be considered a SAE 20 viscosity. Consequently, if a motor oil flows through faster than 9.3 and slower than 12.5 seconds, then it will be a SAE 30 viscosity.
Now if you look at the table labeled Winter or "W" Grades, you can get valuable information on how the W or winter grade viscosities are measured. Basically, as shown by the chart, when the oil is reduced to a colder temperature it is measured for performance factors. If it performs like a SAE 0 motor oil at the colder temperature, then it will receive the SAE 0W viscosity grade. Consequently, if the motor oil performs like a SAE 20 motor oil at the reduced temperatures (the scale varies - see the chart), then it will be a SAE 20W motor oil.
If a motor oil passes the cold temperature or W (winter grade) specification for a SAE 15W and at 210° F (100° C) flows through the viscometer like a SAE 40 motor oil, then the label will read 15W-40. Getting the picture? Consequently, if the motor oil performs like a SAE 5 motor oil on the reduced temperature scale and flows like a SAE 20 at 210° F (100° C), then this motor oil's label will read 5W-20. And so forth and so on!
