What is a GL Number?

What is a GL number, and why is it important? The GL number denotes the amount of protection the oil offers, and simply put, the higher the number, the more protection it offers. The range is from GL-1 to GL-6, with an additional grade being called MT-1.

Without a sufficient GL number, the oil will not be resistant enough. This is due to the higher GL numbers having higher levels of EP (extreme pressure) additives. The EP additives help to provide a barrier which stops the gears touching each other. Using the wrong GL grade can cause the teeth in a transmission to wear away each other as there is metal on metal contact, instead of a thin film of oil. Another issue with using the wrong GL Grade is that the synchroniser rings will not bite.

Currently the active Grades are GL-4, GL-5, and MT-1. GL-1, GL-2, and GL-3 have now been superseded in strength / protection by GL-4 and GL-5. GL-6 is classed as obsolete as GL-5 oils now exceed the GL-6 specification.

GL-1
Designed for use in low pressure conditions this is can be a pure untreated oil. EP additives, and friction modifiers can't be added, however defoamers, pour depressants, oxidation and rust inhibitors can be added. 

GL-2
Very similar to GL-1 however some anti wear and mild EP additives can be used to improve film strength. Designed for use when GL-1 doesn't provide adequate protection for the gears. Also for use with worm gears.

GL-3
This was intended for manual transmissions with moderate/severe load, speed and conditions. It can also be used for spiral bevel axles under mild/moderate loads and speeds. It doesn't meet the load carrying capacity of the GL-4 oils, but supersedes that of GL-1 grade oils.

GL-4
For use mainly in synchronised manual transmissions. This provides good gear protection and still allows synchroniser rings to work. It can also be used in hypoid axles under moderate load, speed and conditions. 

GL-5
Designed primarily for axles operating under high load and high speed. Can handle shock loading and has a higher load rating than GL-4

GL-6
An outdated spec intended for gears with very high pinion offset. Currently obsolete due to modern GL-5 oils having greater levels of EP strength, and the lack of high pinion offset gears being produced. 

MT-1
For use in non-synchronised manual transmissions in heavy duty trucks and buses. It has different levels of protection to those of GL-4 and Gl-5 grades. 

You should always follow the recommendations of the transmission manufacturer when it come to choosing which GL grade to use in your gearbox or axle.