The Sae 300 standards, issued in 2021, introduced an update to the viscosity classification system for engine oils. This year, a similar update has been applied to SAE J306 standards, which regulate lubricants for transmission oils, axles, and differentials, aiming to support the development of higher-efficiency systems
The acronym “Sae” stands for “Society of Automotive Engineers”, the international standards organization serving the aerospace, automotive, and vehicle industries. Among its various responsibilities is the definition of lubricant classification standards for these sectors, which are periodically updated as technological progress gives rise to new or more efficient functional systems—from engines to transmissions, axles, and differentials.
In this context, the SAE has always focused particular attention on the issue of lubricant viscosity, especially on how it changes with temperature.
Viscosity is a physical property that expresses a fluid’s internal resistance to flow, a measure of energy dissipation that varies not only between oils but also depending on temperature, flow speed, and pressure.
Because viscosity is such a variable property, SAE developed the “Viscosity Index”, a parameter that measures how an oil’s viscosity changes as temperature varies. For engine oils, the index is defined according to ISO 2909:2002, which assesses kinematic viscosity at 40°C and 100°C.
The higher the index, the more stable the oil remains across temperature changes. In practice, engine oil viscosity is evaluated through cold tests (representing engine start-up) and hot tests (representing normal operating conditions).
The results are communicated to end users through specific designations made up of two numbers separated by the letter “W”—an abbreviation of “Winter”.
- The first number indicates cold viscosity,
- The second number refers to hot viscosity.
These two values help users—within the viscosity ranges recommended by manufacturers—choose the appropriate lubricant for the climatic conditions in which their engine will operate.
Engines working in cold climates should use oils with low first numbers (more fluid when cold), while those operating in hot climates require oils with higher second numbers to maintain protection at elevated temperatures.
This classification system does not apply only to engine oils—it also extends to lubricants for transmissions, axles, and differentials. These products have recently been redefined under the updated SAE J306 standard, which revises viscosity classifications to support the development of modern, high-efficiency transmissions.
The new revision, following a similar process in 2019, enables the classification and commercialization of lower-viscosity lubricants—that is, more fluid oils—without compromising mechanical protection. The goal is to reduce energy losses and therefore fuel consumption.
This update introduces an advanced concept of low-temperature viscosity, setting a maximum viscosity limit at –40°C, and defines four new low-temperature viscosity grades: 62W, 63W, 64W, and 65W—each with its own maximum viscosity threshold.
These new grades fall below the previous minimum values, offering a more precise classification suited to the requirements of lubricants designed for extreme operating conditions and for next-generation transmissions, such as those found in electrically powered vehicles.
It is important to note that these updates are not just a technical refinement but a concrete response to the growing demand for lower CO₂ emissions and improved energy efficiency.
A more fluid oil allows the transmission to operate with reduced energy loss, enhancing overall efficiency and reducing environmental impact. Moreover, greater fluidity aligns perfectly with the needs of electric powertrains, which are particularly sensitive to friction caused by viscous lubricants at low temperatures.
Of course, this approach requires that reduced viscosity be accompanied by higher oil quality, ensuring stability, performance, and protective capability for all lubricated components.
Ultimately, the new SAE J306 update marks a key milestone in the evolution of transmission lubricants, as the new low-temperature viscosity grades reflect a technological leap that integrates the goals of energy efficiency, sustainability, and compatibility with advanced transmission systems—paving the way for a new generation of high-performance fluids.
Title: Transmission oils: new Sae standards come into force
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