Hi all,
What grade gearbox oil should I use in my 240K box?

It is an old box, so if the gears crunch, you could try some Redline MT-90, good stuff (but it will cost you $60 to fill the gearbox)
eric

Use 75w90 manual trans oil, do NOT use universal diff/gear box oil in datto trans. Stuffs the synchros

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A little knowledge is a dangerous thing.
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"R.I.P. Baz. 29 April 2022. Thank you for all your contributions to the Datsun community over the years. You will be missed." - OZDAT

Why do you say that Baz? What is in a datto box (and apparently not in any other box) that is going to be effected by a slightly different oil? To my knowledge the only thing that stuffs synchros is bad gear changes or too much torque!

Synchros are designed to wear and have a limited life span anyway. They are supposed to be replaced. Hence, a gearbox is a maintainable item. The properties of a combined diff/gearbox oil should not effect the performance of a synchro any more than a specific gearbox oil. The performance of any given oil has everything to do with the pressure (molecular structure strength), viscosity and temperature properties of the individual oil.

Scott, I don't believe a universal diff/gearbox oil will stuff the synchro's so to speak, but I do think that putting in an oil specifically designed for the job is a good idea. Hence, you should put in a manual trans oil, unless it is a competition gearbox. Which is another story.

:shock:Ok actually it dosnt stuff the synchro just stops them doing the job, which i guess is much the same thing. I havent a copy of the Nissan service bulletin warning about the use of other than manual Transoil.
I am of the understanding that the, hope i have the right name here, "Thio-hypoid"? additive that is used in diff oil, to prevent scuffing of the crown wheel & pinion, is present in universal trans/diff oil. This additive plates the synchro rings and reduces their braking action. Many, times i have seen Nissans, mainly with warner type, that have crunching synchros cured just by changing the trans oil to the correct type. It takes about a week or 2 for the box to come good again.

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"R.I.P. Baz. 29 April 2022. Thank you for all your contributions to the Datsun community over the years. You will be missed." - OZDAT

Fair enough, but why are the synchro's in a subaru wrx or toyota celica GT-4 gearboxs, etc (which have LSD's and open diffs in the same case) not effected by this problem? Have Subaru and Toyota etc accounted for this property in Diff oils when designing their gearbox's?

My understanding of a synchro is that it relies on the viscous nature of the oil to accelerate/decelerate the engagement mechanism (before engagement) during a gear change, hence it can be affected by oils with different properties. So I understand that it can be affected by a diff oil but why not other gearbox's?

I guess they have compensated for this but im not that up on subaru/toyota boxes. It may be their synchros use oil viscosity to assist the action. But i guess it depends on the type of synchro system.
My understanding is the warner type as used in Nissans relys on friction.
The porche type servo, as used in Z boxes and some others, certainly uses friction to energise the brake band. For competition use with these boxes,we used ATF for faster/positive shifting.

_________________
A little knowledge is a dangerous thing.
www.datsport.com

"R.I.P. Baz. 29 April 2022. Thank you for all your contributions to the Datsun community over the years. You will be missed." - OZDAT

so this applies to all nissan/datsun boxes including my fj box?
what is the best oil for the job u suggest?

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you are not a beautifull and unique snowflake...

My Synchro's actually behave better once the box has warmed up. (SR20DET gearbox). I'm running a synthetic Penrite 75W-90 gearbox oil. So would assume the viscosity has decreased (oil has thinned) which would squeeze out of the synchro quicker?
I used to have it crunch going into 4th on the race track, but it seems to have fixed itself (probably broke all the teeth of the synchro! - Bahahahah)

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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Got any pictures of your K Scott?

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Godzilla's Grandpa in my driveway!

Seeing as he has no profile, and his original post is 11 years old.
I think Scott's long gone.

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When the sign says "don't feed the bears", man you better not feed the bears.