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)