Time to change gearbox oil?

[MattBLancs]

Just a quick dig, some interesting snippets from Wikipedia:

GL-5 is not necessarily backward-compatible in synchro-mesh transmissions which are designed for a GL-4 oil: GL-5 has a lower coefficient of friction due to the higher concentration of EP additives over GL-4, and thus synchros can not engage as effectively, unless a specialized friction modifier has been included within the oil's additive package; synchro-mesh compatibility is usually explicitly stated and such oils are often known in the trade as TDL (Total Drive Line) oils

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The higher an oil's GL-rating, the more pressure can be sustained without any metal-to-metal contact taking place between transmission components.

Fully synthetic gear oils are also used in many vehicles, and have a greater resistance to shear breakdown than mineral oils.

API Category GL-4 designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications.

API Category GL-5 designates the type of service characteristic of gears, particularly hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Lubricants qualified under U.S. Military specification MIL-L-2105D (formerly MIL-L-2015C), MIL-PRF-2105E and SAE J2360 satisfy the requirements of the API GL-5 service designation

https://en.wikipedia.org/wiki/Gear_oil# ... compounds.


Nothing definite about copper as yet, will dig some more later

[Estately]

Right brain awoken and treated to oaty bungles for breakfast.
Ryman lubricants of Holland states what I dragged out of my memory.

Namely that GL 5 contains higher sulphur and phosphorus content and can react aggressively towards bronze and copper.
Therefore GL 5 can be harmful to synchro rings or similar components in the gear box and GL 4 is recommended.

However the manufacturer's may allow GL 5 and that is the question, hopefully those on this brilliant site may be privy to the necessary information.
Perhaps the oils and lubricants section could be modified accordingly.

Hope that helps.
Andy.