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What Oil In The Gearbox

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What Oil In The Gearbox

Postby WelshBoy » Thu Oct 24, 2019 5:24 pm

Hi All
Just about to refill my (manual gearbox) with oil the transfer case and gearbox side manual says to use the same oil thats in the engine 15/40 engine oil any advice??
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Re: What Oil In The Gearbox

Postby isuzurob » Thu Oct 24, 2019 6:39 pm

Can run 75w/90 but can be hard to select gears when cold, 5w/30, 15w/40, anything really I've tried alsorts even 80w/90 works if that's all you have
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Re: What Oil In The Gearbox

Postby WelshBoy » Fri Oct 25, 2019 5:26 am

isuzurob wrote:Can run 75w/90 but can be hard to select gears when cold, 5w/30, 15w/40, anything really I've tried alsorts even 80w/90 works if that's all you have


Thanks will give the engine oil a try will be cheaper to use the same oil in the trans and motor.
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Re: What Oil In The Gearbox

Postby geeves » Fri Oct 25, 2019 2:22 pm

Not quite unfortunately. Modern engine oils have friction modifiers which are great in the engine but dont play nice with syncro rings. It wont damage them if its already in there but when cold hard gear shifts are likely. If its in there and working well dont worry. The 3 options for the gearbox are
A: The cheapest oil of the same viscosity as the engine that you can find. Today that is still better oil than was available when the gearbox was designed.
B: 5 speed gear oil. There are a few brands on the market. Dont worry about the viscosity as gear oil is measured differently
C: Dexron 3 ATF However it likes to find ways out of the box
All the above work. One thing to avoid at all costs is diff oil. Diff oil additives eat brass synchro rings and break down at high temperatures into nasty things
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: What Oil In The Gearbox

Postby WelshBoy » Sat Nov 02, 2019 4:22 pm

geeves wrote:Not quite unfortunately. Modern engine oils have friction modifiers which are great in the engine but dont play nice with syncro rings. It wont damage them if its already in there but when cold hard gear shifts are likely. If its in there and working well dont worry. The 3 options for the gearbox are
A: The cheapest oil of the same viscosity as the engine that you can find. Today that is still better oil than was available when the gearbox was designed.
B: 5 speed gear oil. There are a few brands on the market. Dont worry about the viscosity as gear oil is measured differently
C: Dexron 3 ATF However it likes to find ways out of the box
All the above work. One thing to avoid at all costs is diff oil. Diff oil additives eat brass synchro rings and break down at high temperatures into nasty things


Thanks have tried the engine oil nylon 15/40 will see how I get on
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Re: What Oil In The Gearbox

Postby Idontwantausername » Tue Jul 21, 2020 6:02 pm

I own a 5 speed bighorn and would recommend 80w-90 gear oil
as Geeves points out gear oil is designed for gearbox's
I tried engine oil in mine and it made getting into 2nd when cold a grawnchy businesses.
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Re: What Oil In The Gearbox

Postby geeves » Wed Jul 22, 2020 9:32 am

Whatever you do never put oil with hypoid additives in the gearbox
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: What Oil In The Gearbox

Postby Kurjaga » Sat Oct 17, 2020 7:29 am

Hello! I have a 2011 Dmax 2.5diesel. I want to know what oil is poured into my manual gearbox? I have trouble switching between 1st and 2nd speeds, but it's only now that it's cold weather. Older models can use the same engine oil that the engine pours, or can it do the same? However, has a technical specification ever been written?
What can you suggest?
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Re: What Oil In The Gearbox

Postby geeves » Sun Oct 18, 2020 6:20 am

Handbook says same oil as engine but this does not take into account modern engine oil additives. These are the cause of your problem. You can use the cheapest engine oil you can find but the recommendations from mechanics here is to use a 5 speed gearbox oil like Castrol VMX or similar. Dexron ATF can also be used
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: What Oil In The Gearbox

Postby handymanny01 » Sun Oct 18, 2020 9:57 pm

I put something along the lines of 75/90 gearbox oil in mine, before I knew it was supposed to be engine oil.... years on,, it's still fine,, defo hard to shift when cold,,,, but my gut feeling is that the true GEARBOX oil, prob provides more protection to the bearings and gears, Especially on a modified engine ... so it's staying in there! If I was standard (un-modified) engine,,, I'd prob put the factory specified engine oil in...
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Re: What Oil In The Gearbox

Postby geeves » Mon Oct 19, 2020 6:28 am

One big word of warning about 75/80 gearbox oil. First and least important gear oils rate viscosity differently to engine oil and 75/80 isnt much different to 30/40 engine oil. More importantly there are 2 75/80 gear oils. One is designed for gearboxes and this is good oil. The other is labeled as hypoid or extreme pressure. This contains a hypoid additive which is great in the diff but it breaks down if it gets hot turning into acid. Also it corrodes brass and syncro rings are made of brass. Do not use hypoid oils in gearboxes except if specified in the handbook. This is not the case with light Isuzu diesels.
Sanding your knuckles before starting work can help. That way you cant skin them
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