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Discussion Starter #1 (Edited)
Sometimes when briskly dropping a cog to 4th from 5th on the motorway, I get a crunch on the downchange.

I have had similar issues with other gearboxes in the past (Evo 5-speed), which have been almost completely cured by using certain types of gearbox oil.

What is a good recommendation for a gearbox oil for my 6-speed gearbox?

One of the best I have used in the past is the Cusco Mission Oil 75W-85 GL3.

Is this suitable for my gearbox and has anyone used it?

Any other recommendations?

The search has recommended various oils, Dexron II ATF, 75W-90, Shockproof etc.etc. - even emails from Getrag stating GL3 75W-90.
 

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redline heavy shockproof will make a big difference but it will also make your gear changes quite stiff as well
 

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I noticed the same on mine when I changed to redline heavy shockproof
Free s a little bit when gearbox is nice and warm but still quite stiff
 

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I used redline heavy shock proof and it was great. Didn't notice a significant stiffening though although this was 7 years ago so maybe a 7 year newer gearbox just felt better anyway
 

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Discussion Starter #7 (Edited)
Sorry yes it's an R34 GT-R, 6-speed Getrag box.

Having read the email from Getrag themselves (in the thread you linked, post #1) saying a GL3 will work, I'm very tempted to try the Cusco 75W-85. I will have to email them the Cusco specs and get another confirmation from them though, I don't want to ruin a good gearbox on an experiment...also the Getrag guy seems to contradict himself if you read both of his emails.
 

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I'd go with Dextron III as that's what almost everyone recommends plus the Supra lot use the same
 

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Discussion Starter #9
OK thanks, I will try that first it's a lot less expensive than trying the Cusco oil. I have emailed Cusco anyway to see what they say about their oil in this gearbox.
 

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Discussion Starter #10
Looking at some Datasheets I found from Castrol, Transmax Z is recommended by Nissan as an alternative to Nissan Matic Fluid D. Do Nissan fill the R34 GT-R with Matic Fluid D?

Comparing viscosity specs at 40 and 100 deg:

Transmax Dexron III: 35mm/s and 7.2mm/s
Transmax Z: 37.8mm/s and 7.5mm/s

So there is hardly any difference in viscosity, and Castrol claim it exceeds the Dexron III specs and is recommended for manual transmissions using Dexron II or III. It can also be mixed with them. On top of that they claim that even though it is fully synthetic, there are no issues in gearboxes which would normally use a mineral based or semi synthetic oil.

Typical usages include racing applications - oh and it is green too.

Well it sounds like that would be the one to go for - any thoughts?
 

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The Castrol Transmax is what the Supra uses I think; it's very expensive and only available in large quantities? I'd have no problem using that myself
 
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