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Important Noob question about Transmission and Differential Oils?

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Old 01-12-2014, 12:26 PM
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Important Noob question about Transmission and Differential Oils?

please dont flame this.

ive search and searched and havent found an answer

im looking to change my tranny and diff fluids and ive purchased 3 bottles of redline MT-90 (GL-4) for my transmssion, which ive heard is good.

My question is that ive read that i need the redline 75W90 (GL5) fluid for my diff and i was wondering if i really need another fluid or can i use the GL4 in the diff without any issues?

any answer is helpful
thank you
Old 01-12-2014, 12:43 PM
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https://www.rx8club.com/new-member-f...ff-oil-249929/
https://www.rx8club.com/series-i-tec...-forum-244954/
https://www.rx8club.com/rx-8-discuss...-fluid-161235/
https://www.rx8club.com/new-member-f...-fluid-248694/

https://www.rx8club.com/rx-8-discuss...0/#post4557040


Lots of search results with the information.


One of the questions and responses recently:
Originally Posted by Felix_Wankel_fan88
can anyone help me understand the difference between MT oil and differential gear oil.

i've been told they are the same thing in my Rx8 (but i don't trust my source)

the manual trans calls for MT90 oil
the diffs call for 75w90 gear oil....is it the same thing?
i've found the 5cats.org walk through, but im confused on the oils... thanks in advance for helpful replies
Originally Posted by RIWWP
They have the same viscosity, but they are different oils with different purposes.

MT-90 (Red Line Synthetic Oil - Gear Oil for Manual Transmissions - MT-90 75W90 GL-4 Gear Oil) is a GL-4 oil, and has friction modifiers to allow syncro operation (they do need to grip stuff), use in the transmission.
75w90 Gear Oil (Red Line Synthetic Oil - Gear Oil for Manual Transmissions - 75W90 NS GL-5 Gear Oil) is a GL-5 oil, and does not have the friction modifiers, use in the rear diff.

Using a GL-5 in the transmission will destroy your syncros in a few thousand miles (lots of examples on the forum from years back). Using a GL-4 in the rear diff will likely destroy the diff in a few thousand miles (though I don't know of any reported cases where it did happen)

(An expert on oils might correct me, if i have any of that wrong)
Old 01-12-2014, 12:49 PM
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thank you so much.
this is exactly what i needed to know
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