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Gear Oil with API GL-4
API GL-4 is the category for manual gearboxes with brass or bronze synchroniser rings: enough extreme-pressure additive for the tooth flanks, little enough not to attack the soft metals. If your maintenance schedule calls for GL-4, a straight GL-5 is no substitute.
Why GL-5 harms a synchromesh gearbox
Both categories contain sulphur-based extreme-pressure additives, GL-5 in a considerably higher concentration, because hypoid gearing in a final drive works under far greater contact pressure. These additives act chemically, not mechanically: they form a reaction layer on the metal surface that prevents scuffing.
On steel that is exactly what you want. On the brass and bronze synchroniser rings of a manual gearbox, the same chemistry attacks the material. The wear is gradual and shows up as a graunch on fast shifts or a notchy second gear when cold - by which point the cause can no longer be undone.
GL-4/GL-5: the tested dual approval
Oils carrying both categories on the container have expressly been tested for compatibility with soft metals and may be used in both places. That is something different from a straight GL-5 poured into a gearbox "as well" - the difference is on the label, not in the viscosity.
The viscosity you choose separately
GL-4 comes as 75W-80, 75W-85 and 75W-90. Modern gearboxes are more often specified for the thinner grades, because cold shifting is what the driver notices. The category says nothing about the viscosity, and the viscosity nothing about the category: you need both figures from the maintenance schedule.
Frequently asked questions
Can I use GL-5 where GL-4 is required?
Only if the container expressly says GL-4/GL-5. A straight GL-5 attacks brass and bronze synchroniser rings over their service life; the damage shows up as graunching shifts and cannot be reversed.
Is GL-5 the better category?
No, the different one. GL-5 is intended for hypoid axles with higher contact pressure, GL-4 for synchromesh gearboxes. A higher number here does not mean higher quality, just a different place in the drivetrain.
How do I know which category my gearbox needs?
From the maintenance schedule: it states the API category and often a brand-specific approval number as well, which then has to be findable on the container. For final drives GL-5 is the normal case, for manual gearboxes GL-4.
