our tranny has a lsd in it doesn't it
The rear differential on many of the GT-Fours and some alltracs came as a Torsen LSD.
The transmission has an Open Front differential, and a Viscous Center Differential.
so, when you say for find a fluid for the job.. talking about the tranny- why worry if it works with a diff?
The differential in our cars has different lubrication requirements than our transmission.
Our manual states that a 75W90 GL5 fluid should be used in both the transmission and differential.
Unfortunately, this is kinda vague. The transmission has syncros in it which do not work well with a GL5 Differential fluid.
From what I understand, syncros work best in a GL4 fluid. Unfortunately, out transmission is also a transaxle, containing both the front and center differential which use the same fluid.
Those differentials ideally need GL5 Fluid. So, a comprimise has to be made.
This is where Redline comes in with their multi application oils
They made a GL5 75W90 specifically for hypoid differentials (like ours)
They also make a GL5 75W90NS formulation which protects like the 75W90, but without the "Slipperyness" needed for the differential.
This slipperyness is counter productive to the operation of the syncros in a transmission.
But to further confuse matters, I have also read that the Shock protection chemical of GL5 fluid can have a negative effect on "yellow metal" syncros like in our transmission.
Redline markets their 75W90NS as being syncro safe: From their site:
[75W90NS] Contains extreme pressure additives like our 75W90 GL-5 oil, but lacks friction modifiers to balance slipperiness.
[Plus] Improved copper corrosion protection to prolong synchro life
So... Its commonly recommend to use Redline 75W90 NS in the transmission, and straight 75W90 in the rear differential.
Now, from my previous post, i mentioned a redline rep suggested i use their GL4 MT-90 75W90 fluid in my transmission.
This would be great for the syncros from what i understand... but i dont know if the GL4 will provide the same protection for the front and center differentials.
I asked him for specifics on why he suggested that specific oil. I hope to get a response soon.
If anything i have posted here is incorrect - someone please let us know.
Hope that helps.
There is a large "Transmission and Differential" thread on here summing up what each car came with.