Pulled from the internet:
GL-4 Suitable for vehicles requiring an 80w90 GL-4 Mineral Gear Oil and fully suitable for use where yellow metals i.e. Brass, Phospohor Bronze etc are present. This oil is classed as an EP Gear Oil meaning it contains Extreme Pressure additive. GL4 is the first serious level of EP performance and is specified for many gearboxes and final drives from mid 1930’s onwards.
GL1 is the lowest level of performance and is only used in vintage transmissions. It is very similar to engine oil, having a very low additive content, but is available in much higher (thicker) viscosities to suit gearboxes with unsophisticated oil seals.
https://penriteoil.com.au/knowledge-cen ... -oils/1036
Originally, lead additives were used in gear oils and were very good at reducing wear in loaded gear sets. Lead though, is not very friendly to the environment and was phased out (after WW2 TM manuals were written) in favor of a Phosphorus and Sulphur combination. These gear oils used the Phosphorus and the Sulphur to attach to the gears and create a strong sacrificial layer to be worn off over the life of the fluid, thus protecting the gear from abrasive wear, high load and shock loads. Initially, the problem with this type of formulation was that the Sulphur used was active and caused corrosion of yellow and other soft metals used in transmissions and differentials. This happened because active sulphur reacted with some metals and metal alloys, especially those that used copper as the alloying element to form metal sulphides that in turn caused corrosion of the metal.
A generation ago deactivated or buffered sulfur was developed that could react with the phosphorous and create a protective and sacrificial layer in conditions created inside gear boxes. This formulation was not corrosive to brass, copper or other metal alloys used in transmissions. Today this is widely used in automotive transmission and gear oils.
Under normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of the softer metal.
An API GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive that would be in the API GL-5 product, so the bond with the metal surface inside the transmission is not as strong, and therefore can be peeled off without peeling a layer of soft metal. This means that the GL-4 product provides a little less extreme pressure protection than a GL-5 oil but less wear on synchromesh components of a transmission. When a GL-5 oil is used in a transmission with synchromesh it can create up to 4 times the amount of copper in a used oil analysis as that of a GL-4 product.
