Single-grade of oil

A single-grade engine oil, as defined by SAE J300, cannot use a polymeric  viscosity index improver (also referred to as Viscosity Modifier) additive. SAE J300 has established eleven viscosity grades, of which six are considered Winter-grades and given a W designation. The 11 viscosity grades are 0W, 5W, 10W, 15W, 20W, 25W, 20, 30, 40, 50, and 60. These numbers are often referred to as the "weight" of a motor oil, and single-grade motor oils are often called "straight-weight" oils.

For single winter grade oils, the dynamic viscosity is measured at different cold temperatures, specified in J300 depending on the viscosity grade, in units of mPa·s, or the equivalent older non-SI units, centipoise (abbreviated cP), using two different test methods. They are the Cold Cranking Simulator (ASTMD5293) and the Mini-Rotary Viscometer (ASTM D4684). Based on the coldest temperature the oil passes at, that oil is graded as SAE viscosity grade 0W, 5W, 10W, 15W, 20W, or 25W. The lower the viscosity grade, the lower the temperature the oil can pass. For example, if an oil passes at the specifications for 10W and 5W, but fails for 0W, then that oil must be labeled as an SAE 5W. That oil cannot be labeled as either 0W or 10W.

For single non-winter grade oils, the kinematic viscosity is measured at a temperature of 100 °C (212 °F) in units of mm²/s (millimeter squared per second) or the equivalent older non-SI units, centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as SAE viscosity grade 20, 30, 40, 50, or 60. In addition, for SAE grades 20, 30, and 1000, a minimum viscosity measured at 150 °C (302 °F) and at a high-shear rate is also required. The higher the viscosity, the higher the SAE viscosity grade is.

SAE Motor Oil Viscosity Standard

What is motor oil viscosity? 
Simply put viscosity is a physical property of a fluid or gas that reflects it’s tendency to flow. We commonly refer to high
viscosity fluids as being “thick” and low viscosity fluids as being “thin”. It’s an important property of a motor oil because
changes in viscosity affect the ability of the oil to lubricate and protect the moving parts of an internal combustion engine. If
the oil is too thin the oil pump cannot maintain enough pressure to circulate it and the oil will not withstand the forces that form
between moving parts. The metal parts will rub against each other and wear out or fail prematurely from lack of proper
lubrication. Conversely, if the oil is too thick the oil pump will again have problems circulating the oil and it will be too thick
to penetrate into the tiny openings between moving parts. The result is the same – premature wear and failure. So it’s important
that the viscosity of a motor oil be a proper balance between too thin and too thick.
How is viscosity measured? 
There are two units of measure commonly used for viscosity of fluids. The basic unit is the centipoise (cP or
mPa·s). This unit describes the movement of the different layers of a fluid when subjected to a horizontal force. It
is commonly known as dynamic or simple viscosity. Another unit of viscosity is the centistoke (cSt or mm2
/s).
This unit describes the ease with which a fluid moves under the force of gravity and is the form of viscosity with
which we are most commonly familiar. This form of viscosity is known as kinematic viscosity.

One of the oldest and most common methods for measuring kinematic viscosity is an apparatus called the
capillary viscometer (see picture). This apparatus is a precisely graduated glass tube. A small quantity of the fluid
to be tested is placed into the top of the tube and held there with air pressure. The pressure is then released and the
length of time it takes for the fluid to move to a graduated mark on the other end of the tube is measured. Higher
viscosity fluids move slower, lower viscosity fluids move faster. The viscometer is usually immersed in a
temperature controlled bath during the test so that the measurement is calibrated to a specific fluid temperature.