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Found an interesting article on GL-4:

API to Retire GL-4 Spec
By Stephen Swedberg

On July 1 the American Petroleum Institute polled its Lubricants Group on declaring the GL-4 gear oil category a “service designation not in current use.”

If approved, API GL-4 will become obsolete on July 31.

API GL-4 has been a part of the gear oil family for many years, denoting lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid gears operating under moderate speeds and loads.

Although this service designation is still used commercially, some test equipment used for performance verification is no longer available.

At the May 5 meeting of the API Lubricants Group, Kevin Buzdygon from ExxonMobil Research and Engineering recommended that GL-4 be made obsolete because it is impossible to run most of the required tests.

Currently, according to the latest (1995) edition of API publication 1560, “Lubricant Service Designation for Automotive Manual Transmissions, Manual Transaxles, and Axles,” said Buzdygon, the following are active industry specifications for automotive gear oils:

– API GL-1 : mildly loaded gearing, products are just base oil (no test requirements).
– API GL-4 : moderately loaded gearing (original tests listed in ASTM STP 512A-1986).
– API GL-5 : highly loaded and/or high speed gearing (now ASTM D7450).
– API MT-1 : heavy duty non-synchronized manual transmissions (now ASTM D5760).

GL-4 is most often used in older gear sets. Some of them are truck applications where the loads are not very heavy as well as older automobiles. GL-4 gear oils also found some applications in industrial gear sets that were very heavily loaded but were not offset.

As car designs went to lower profiles and before front wheel drive became common, rear axles had to go to very high offset hypoid gears. That results in some terrific loads on the gear teeth as the power is transmitted to the rear wheels from the engine. GL-5 became the oil of choice with those designs. GL-5 was always the choice for heavy duty trucks and off-road vehicles since the shock loads and wear are so great in those applications.

MT-1 is a relatively recent addition and is specifically designed for manual transmissions. It has load carrying capabilities similar to GL-4 but has extra wear, corrosion and antifoam capabilities to deal with the transmission.

Buzdygon noted that most API GL-4 tests listed in ASTM STP 512A have been unavailable for over 14 years. With the exception of some older additive technologies, API GL-4 quality claims cannot be supported as originally intended by ASTM STP 512A. This means that an alternate means of supporting API GL-4 quality claims would be necessary.

As far back as 1987, at least one test was known to be unavailable and the category was considered obsolete, Buzdygon continued. Since that time, a new specification has remained unsettled within ASTM.

Without access to the required tests, additive suppliers and lubricant marketers tend to use one or both of the following methods to support API GL-4 claims, said Buzdygon.

Under the “50 percent treat rate” approach, if API GL-5 quality can be documented for a given treat rate of an additive package, then half the treat rate of that additive package is deemed to provide API GL-4 quality. This approach is not documented in any API, SAE, or ASTM publication, but it is used frequently in the industry, Buzdygon said, and although this approach may have been valid in the past, being based on actual test data, this approach would not necessarily apply to untested additive chemistry.

A second method is to document performance by running selected tests that are similar to the original API GL-4 tests listed in ASTM STP 512A. However, technical judgment is required to make a proper choice of test method and test results, and varies greatly within the industry, said Buzdygon.

API’s Kevin Ferrick recommended to the Lubricants Group that since it is not possible to run most of the tests required for API GL-4 the specification should be made obsolete. The consequences of making API GL-4 obsolete are minimal and public recognition of API GL-4 claims is not expected to change very much, he noted. Lube marketers may need to adjust the language used to make API GL-4 claims and the “50 percent treat rate” approach should not be used without a reasonable level of additional supporting documentation.

In 2006, the latest year for which reliable data are available, U.S. gear oil sales totaled 33 million gallons, according to the National Petrochemical and Refiners Association.
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The retirement announcement does not eliminated the use of it or the need for it. It was retired because the test equipment didn't exist any more to prove compliance.
Also considered throughout 15 years of debate on the issue was the fact that many manufacturers have gone with their own spec to keep people from using GL-5, or come out with their own "Synchromesh" or manual transmission fluid. Each actually would meet the GL-4 EP requirements (but can't be proven).

I agree with Chevron's dissenting opinion that the whole transmission/differential classification needs to be revamped.

Yes, officially GL-6 has been retired for the same reason, but some brands still spec it for their differentials.

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