The SAE (Society of Automotive Engineers) is responsible for the SAE classes of engine oils (SAE J 300) as well as for the viscosity classification of automotive gear oils. SAE J 306, revised by this committee at the end of 2005, is slowly being applied in practice. There are initial requirements for SAE 110 or SAE 190 oils, and the first oil manufacturers are stocking these lubricants.
It should be noted that the SAE for gear oils is not a continuation of or a supplement to the SAE for engine oils. Despite the higher numerical value for the SAE class, an SAE 80 is not twice as "thick" as an SAE 40. The viscosity values of the two tables have no common logic. One table is valid for engine oils, the other for gear oils. High- and low-viscosity oils are equally represented in both tables
The need for a revision of the SAE classes for gear oils arose from the wide viscosity range of the "old" SAE 90 (18.5 cSt to <24 cSt at 100° C) and the former SAE 140 (24 cSt to < 41 cSt at 100° C). This meant that the actual viscosity of oils could differ greatly, even though they fell into the same SAE class. The two new viscosity classes SAE 110 and 190 were introduced to delineate the respective upper ranges of these old classes. The resulting more narrowly defined SAE classes, which can now also be more easily assigned to the ISO VG commonly used for industrial oils, allow gear manufacturers greater flexibility in specifying viscosity. This makes it easier to optimize gears in a way that strikes a balance between fuel consumption and gears durability.
The use of an oil that is too "thin" often has a negative effect on the wear behavior and thus the service life of a gear. Oil that is too "thick" will result in increased fuel consumption or it may cause stiffness when shifting gears.
When selecting a gear oil according to the predecessor SAE, it could happen that an axle was filled with an SAE 90 oil that had a significantly lower viscosity (e.g. 13.5 mm2/s) than the gear oil (e.g. 22 mm2/s) originally used for testing. In order to prevent an axle from being filled with an SAE 90 oil that was too thin, the gearbox manufacturer was often forced to specify a higher viscosity (e.g. SAE 85W-140 with a viscosity of usually approx. 26 mm²/s) than would actually have been necessary. Today, SAE 110, for example, can be used in such cases