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Discussion Starter · #1 ·
I have a 1992 SJ413 (in Thailand - long wheel base). I noticed that most of the gear oils on sale here whether synthetic or non-synthetic have an API classification of GL-5. I cannot find a GL-4 rating except Redline's MT-90 and MTL which are available here. I looked at the old manual of the jeep and all it says for the gear oil is "SAE 90".

I read in the internet that a GL-5 rating gear oil has "active extreme pressure additives that may contribute to chemical corrosion, etching or wear of yellow metals such as brass or bronze". Is this something I have to be careful about with regards to the synchros and gears in my trany? I am thinking of using the Pennzoil SAE 75W90 synthetic GL-5 rating. Redline's MTL and MT-90 which are available are both rated GL-4. Any recommendations? Or the API classification doesn't matter?

Thanks a lot.

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