SAE J300 is a standard that defines the viscometric properties of mono- and multigrade engine oils, maintained by SAE International. Key parameters for engine oil viscometrics are the oil's kinematic viscosity, its high temperature-high shear viscosity measured by the tapered bearing simulator, and low temperature properties measured by the cold-cranking simulator and mini-rotary viscometer. This standard is commonly used throughout the world, and standards organizations that do so include API and ILSAC, and ACEA. The SAE has a separate viscosity rating system for gear, axle, and manual transmission oils, SAE J306, which should not be confused with engine oil viscosity. The higher numbers of a gear oil (e.g., 75W-140) does not mean that it has higher viscosity than an engine oil 20W-50.
Grades In the SAE J300 standard (2021), the viscosity grades are 0W, 5W, 10W, 15W, 20W, 25W, 8, 12, 16, 20, 30, 40, 50, and 60. In the United States, these numbers are often referred to as the "weight" of a motor oil, and single-grade motor oils are often called "straight-weight" oils. The grades with a W designation are considered Winter-grades, and denote an engine oil's low-temperature properties, while non-winter grades denote an engine oil's properties at the operating temperature of an engine. The SAE 8 through SAE 16 viscosity grades describe oils that can improve fuel economy through reduced hydrodynamic friction.
To assign winter grades, the dynamic viscosity is measured at various cold temperatures, specified in J300, in units of mPa·s, or the equivalent older non-SI units, centipoise (abbreviated cP), using two test methods. They are the cold-cranking simulator (CCS, ASTM D5293) and the mini-rotary viscometer (pumping, ASTM D4684). Each temperature is associated with a grade, SAE 0W, 5W, 10W, 15W, 20W, or 25W, with higher grade numbers corresponding to higher temperatures. The oil fails the test at a particular temperature if the oil is too viscous. The grade of the oil is that associated with the coldest temperature at which the oil passes the test. For example, if an oil passes at the specified temperatures for 10W and 5W, but fails at the 0W temperature, the oil is grade 5W. It cannot be labeled 0W or 10W.
To assign non-winter grades, kinematic viscosity is graded by ASTM D445 or ASTM D7042, measuring the time it takes for a standard amount of oil at a temperature of 100 °C (212 °F) to flow through a standard orifice, in units of mm2/s (millimetre squared per second) or the equivalent older non-SI units, centistokes (abbreviated cSt). The longer it takes, the higher the viscosity and thus the higher the SAE code. Larger numbers are thicker. J300 specifies a viscosity range for each non-winter grade, with higher grade numbers corresponding to higher viscosities. In addition, a minimum viscosity measured at a high temperature and high-shear rate (HTHS, ASTM D4683) is also required.
Multi-grade designations Grades may appear alone - for example, a lawnmower may require SAE 30. This single grade specification means that the oil must meet the SAE 30 requirements. But SAE also allows designating an oil with two viscosity grades, referred to as a multi-grade oil. For example, 10W-30 designates a common multi-grade oil. A 10W-30 oil must pass the SAE J300 viscosity grade requirements for both 10W and 30, and all limitations placed on the viscosity grades, such as the requirement that a 10W oil must fail the 5W requirements. Viscosity index improvers (VIIs) are special polymer additives added to oil, usually to improve cold weather performance in passenger vehicles. If any VIIs are used, the oil must be labeled with a multi-grade designation. Otherwise, an oil not containing VIIs can be labeled as multi-grade or single grade. For example, a 20W-20 oil can be easily made with modern base oils without any VIIs. This oil can be labeled as 20W-20, 20W, or 20.
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