Gear Oiil Information

[WarStryker13]

The most recommended gear oil for transmissions with brass synchros is Redline MT-90, and there's the Amsoil Manual Transmission & Transaxle Gear Lube 75W-90 that is well recommended, but most will probably use whatever they find on the shelf in store. That can easily end up in harming the transmission and causing problems. I have done some research and want to pass on the info to save somebody from needing a rebuild or new transmission.

You should know that some API GL-5 gear oils will damage the brass synchros in the manual trans because of the sulphur and phosphorus additives, so the recommendation is to use API GL-4 gear oils. I found a great technical explanation of what happens when you put GL-5 gear oil into a manual trans with brass/bronze (yellow metal) components: https://www.widman.biz/uploads/Transaxle_oil.pdf

There's a lot of info on this forum, and these threads in particular:
https://www.bobistheoilguy.com/forum...Number=2554764
https://www.bobistheoilguy.com/forum...Number=1674043
https://www.bobistheoilguy.com/forum...Number=1078234

This report gives a lot of insight into what your favorite gear oil brands will do to the insides of your transmissions, transfer cases, and axles: http://www.goodsenseoil.com/G2457-GearOilWhitePaper.pdf

I will be using either Redline MT-90 or Amsoil MTG 75W-90 in my 5 speed, because it's got 211K miles and I want it to continue to shift flawlessly. However, these gear oils are $18-$20 per quart, and that adds up quickly. I'm going to use the same gear oil in the transfer case, because the factory specifies the same gear oil for both and these two gear oils are the best possible options for our application. That's just under 5 quarts total, but I consider it to be money well spent.

There are cheaper options: Sta-Lube 75W90 GL-4 and Nissan 75W85 MTF, and some GL-5 gear oils are not as damaging to brass. If you change your gear oil often and regularly, you can most likely get away without any real damage to the brass components in your transmissions.

And for your differential lube, it's perfectly safe to use modern GL-5 gear oil. It's designed specifically to handle the load and pressure exerted on the oil by the differential gears and ring & pinion gears during both normal operation and in heavy duty/high horsepower applications.

Just be mindful of what you're putting in your trucks.