moved to Under The Hood

 

You're talking about the differential and meant to say gear oil, right? Because gearbox is slang for a transmission.

 

moved to Under The Hood

 

you shouldnt be using any type of gear oil in the tranny...75w90 and 75w140 is entirely way too thick for the trans

 

Do a search and you will find some members on here that use 75/90 in the tranny.
I use a 50/50 mix of M1 75/90 and 75/140 in the rear end(use this because I use that mix in all my cars(3) and two bikes.
Have never had a problem in any of my vehicles with this mix(been doing this for 30 years).

ROD

 

thats scary then being since Honda MTF i believe is equivalent to 10w30...personally i run syncromesh and never had a problem and i think thats closer to 5w30 in weight...i could be wrong, but i just feel like running a fluid that thick cant be good

 

I've used Redline MTF, viscosity 75w90, in both the S2000s that I've owned. That MTF appears to be about the same viscosity as 10w-30 motor oil. Here's some data re that MTF:

Satisifies the gear oil viscosity requirements of 70W, 75W, 80W, and motor oil viscosities of SAE 30, 10W30, and 5W30

 

Not sure about drivetrain loss. I use Honda MTF for the tranny and have used 75w90 for the diff, although I'd like to try Amsoil 75w110 next time. Didn't really think about mixing 75w90 & 75w110 though.

 

TLDR; Use MTF in the trans, and 75w90 or 75w140 in the diff. I use Honda MTF and Mobil 75w90(LS) because that's what Billman250 uses and if it's good enough for him, it's good enough for me.

I'm no oil expert and this from the top of my head (so some GL-2 & 3s and GL-4 & 5s might be mixed) but from what I've read...
The transmission uses helical gears (top image) and the differential uses hypoid gears (bottom image).



The recommended oil for the transmission is Honda MTF and for the differential, it is API GL-5 75W90 gear oil.
As far as the comparisons of weight, Honda MTF is similar to 10W30 motor oil and 75W90 gear oil is similar to 10W40 motor oil. The difference between the two isn't huge, but I felt like there was a difference when I was using my gear oil pump for them.

However, the biggest difference between the two oils is the additives. The transmission benefits more from anti-wear additives (MTF, GL-2, GL-3) and the differentials more from the anti-scuff additives (GL-4, GL5). (For instance, the Subaru BRZ manual recommends 75w90 GL-3.)

What are you trying to gain from running 75w140 in the transmission?