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Discussion Starter #1
I just rebuilt the transmission of my 88, still has the stock 5 speed. I replaced all bearings and synchros. I know GL-5 is whats widely available but I also know yellow metals dont care for it much.

Some say its also to slippery and wont slow things down enough for a smooth shift.

Unless something else works really well and has served over the long term (yellow metal safe) Im leaning toward some GL-4.

I want my rebuilt transmission to last a good long time and shift as well as it possibly can.

Brands? Synth or Dino? Where to get it, I didnt find any GL-4 locally.

Thanks folks for any/all input!

Tony
 

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I've been using Mobil One synthetic 75w90 GL-5 for over a decade - I will probably switch to Valvoline SynPower at the next change, but for no other reason than Exxon/Mobil products are no longer available where I live.
 

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GL-5s can have Extreme Pressure additives that can be corrosive to the yellow metals used in the syncros. Some GL-5s, such as Redline 75W90NS, are approved for transmission use though.

It is safest to use GL-4 in the transmission. Suzuki also calls for GL-4 in the transfer case.

I run Redline MT90 in the transmission and transfer case. I run Amsoil Severe Gear 75W90 in the diffs. You should be able to find an Amsoil dealer locally, and Amsoil Manual Transmission & Transaxle Gear Lube 75W-90 Product Code: MTGQT-EA would be a good fit for the transmission and t-case if you can't find the Redline or don't want to order it.

I noticed a marked inprovement in shifting when switching to the MT90 gear oil. Redline MTL and MT90 have been well-known for improving shift quality for a long time.
 

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Discussion Starter #5
That would indicate that GL-5 is appropriate. I have never changed gear oil several times just to see which viscosity produces the smoothest shifts in a given transmission but have read enough folks saying the GL-4 improved over the GL-5 for shift quality.

At this point Im ready to dump in the GL-5 I have in stock, it says differentials and approved for transmissions, but doesn't say "yellow metal safe" To the best of my knowledge the vast majority of manual gearboxes use brass synchros.

I'mma havta giver a try I think, it is 80-90 and there is really no place local to get GL-4 straight up. I will try for a local Amsiol dealer first, I'm pretty sure I saw a sign in front of a house boasting local Amsoil sales.

Tony
 

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