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GL4 vs GL5 on gearbox.

Draft Horse

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Was having a discussion with a mate about my recent transmission oil change on my 90 series.
Manual says to put either GL4 or GL5, and since I heard GL4 was more recommended for manual gearbox, I put in that. But the mate says I was stupid because GL5 was better, if the manual says I could had also use it without a problem.
What would you use?
 

Chris

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Any oil that said 'gearbox oil' on the can.

GL5 better? In what respect for a 20 year old 4x4 gearbox?
 

Beastrider

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Hi - if the manual says GL4 or 5, then Toyota have already determined that either one is acceptable.
In my opinion, you made the right choice for your gearbox. The main difference between 4 and 5 is the amount of Extreme Pressure (EP) additives - 5 having a lot more in it than 4. Gearboxes don’t need as much additive as axles because the loading on the gear teeth is lower and micro-welding is less likely.
EP are sulphur and phosphorus compounds (which is why gear oil smells the way it does) and they can react with the copper in the bushes and synchro rings, so I think why have more EP additive if it’s not required.
Correct viscosity is important too - not just the grade.
 

Draft Horse

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He said something along the lines of the thicker oil film between the gears with the GL5 oil will better avoid metal to metal contact.
Me personally also believe that those gears don't need those extreme pressure additives, and may even contributive to more wear over the years with the increase pressure between the gear teeth. I'll stick with the normal standard oils, even if Toyota says its fine.
 

Beastrider

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Your mate may have been referring to the thickness of the EP layer that forms on the gear teeth and he would be right that GL5 gives more protection than GL4 for the gear teeth. But those same EP compounds cause erosion of the synchro rings over time. The GL rating only refers to gear protection - it does not consider the life of the synchros.

The manual says both GL4 and GL5 can be used but I believe GL4 is the better choice for the gearbox in normal vehicle usage conditions (especially one that is 20+ years old and inevitably already worn).
 
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