I know absolutely nothing about oil viscosities and stuff like that and am not sure which manual tranny fluid to use: RedLine MTL or MT-90? The FSM says the AX-15 needs an "SAE 75W-90, API quality gear lubricant."

Redline, says their fluids are:

MTL®

- Manual Transmission/Transaxle Lubricant.
A 75W/80W GL-4 gear oil which is designed for manual transmissions which require 75W or 80W GL-4 gear oils, motor oils, or most special synchromesh fluids. It provides low temperature shiftability equal to an ATF, yet will not thin out or shear at high temperatures and provides the substantial gear protection of a GL-4 gear oil. The higher synchronizer coefficient of friction allows faster upshifting and downshifting and the lubricant is non-corrosive to synchros and bushings.

MT-90

- Manual Transmission/Transaxle Lubricant.
A 75W90 GL-4 gear oil which is designed for manual transmissions which require 75W90, or 90 GL-4 gear oils or motor oils. Uses the same chemistry as MTL® , but is a 90 WT in order to satisfy certain manufacturers which recommend SAE 90 GL-4 gear oils.

Does that mean we should be putting the MT-90 in? Is it all that big of a deal? I've already wasted $40 putting some gear oil in that eats my synbchros, so I'd like to do this right the 2nd time. I live in Wyoming where the winter temps get to -20 for a couple weeks and about 100 or so in the summer. Any advice would be greatly appreciated.

Thanks,
s

Oh and for the sake of learning, what is the difference between a 75W-80 and a 75W-90? When would you choose one over another?

 

I used to run MTL in my manual before I became an Amsoil dealer.

I never noticed any problems with my Peugot tranny. Its requirements are the same as the AX-15.

You are going to want to make sure that the fluid meets the API GL-4 specification NOT the GL-5 specification and you will be fine.

When you are looking at lubricant viscosities think of it as scale.. A 10w fluid is thinner at room temps than a 90w...

But a mutli-viscosity fluid (75w-90) works a little bit different. It will be the same viscosity as a 75w fluid at zero degrees, but at operating temps it will be the same viscosity as a 90w fluid.

 

I have used both mt90 mtl in my old tj with ax15 couldn't really tell any difference the mtl is thinner than mt90 if that helps.