The earliest attempts to classify motor oils were made when automobiles first appeared.  Even at this early stage, viscosity was recognized as one of the most important characteristics of an oil.  For this reason the Society of Automotive Engineers (SAE), in co-operation with engine manufacturers, developed a classification system for motor oils based on viscosity measurements.  Oils are assigned numbers based on the viscosities displayed at certain temperatures.

It has more recently been recognized that oil viscosity at colder temperatures, as well as at high operating temperatures, is very important in the long life performance of an engine.  Therefore, the SAE has developed two separate viscosity measurements, one at cold temperatures and one at high temperatures.  The most common temperatures in the high temperature range are 40°C and 100°C.  With the introduction of viscosity measurement at cold temperatures, using a rotating viscometer called a cold cranking simulator; temperatures are reported as low as -30°C.  Because the viscosities are tested in two different temperature ranges, the results are reported in two different units.

The first unit is the centipoise (cP).  It is used to report the absolute viscosity of an motor oil at colder temperatures.  This number indicates the ease with which the oil can be moved.  The other unit is the Centistoke (cSt) which is used to report the kinematic viscosity of motor oil at high temperatures.  The number reflects the time required for a fixed amount of fluid to flow through a certain sized orifice on the testing device.

Oils that are suitable for use in colder temperatures are designated by the letter “W” when indicating the SAE viscosity grade.  These grades of oil must meet maximum viscosities at specified temperatures and also must meet maximum requirements for the borderline pumping temperatures as shown in the table below. Oils that are suitable for use at higher temperatures have viscosities within the ranges also shown in the table below.

VISCOSITY TABLES
ENGINE OIL VISCOSITY CLASSIFICATION

 

SAE VISCOSITY GRADE

 

BORDERLINE PUMPING TEMPERATURE °C

COLD CRANKING SIMULATOR CENTIPOISES (cP) AT TEMPERATURE °C

 

KINEMATIC CENTISTOKES (cSt) AT 100 °C

 

MAX.

MAX.

MIN.

MAX.

0W

-35

3250 at -30

3.8

-

5W

-30

3500 at -25

3.8

-

10W

-25

3500 at -20

4.1

-

15W

-20

3500 at -15

5.6

-

20W

-15

4500 at -10

5.6

-

25W

-10

6000 at -5

9.3

-

20

-

-

5.6

9.3

30

-

-

9.3

12.5

40

-

-

12.5

16.3

50

-

-

16.3

21.9

Note:  1 cP = 1 mPa s             1 cSt = 1mm2/s

Motor Oil Viscosity Excerpt from "The Practical Handbook of Machinery Lubrication".