I know, I know, the standard is 75-90 GL4 (Redline MT-90) I bought mine used with 80k miles. While the trans doesn't feel "problematic", it's not exactly smooth. Curious if ANYONE has tried a heavier oil. Note: I did add URD's short shifter. I understand that might add a little to it, but still wondering.
Here is something I found on bobistheoilguy.com Look up the Article and see if it answers your question.
Looks like a higher viscosity would be a bad move.
Current MTL GL4 viscosites are:
1. ATF Series - Type; 6.5 to 8.5 cSt (Equivalent ATF viscosity; Note: ATF additive package is weak compared to most GL 4's)
2. Synchromesh Series -Type; 9.3 - 9.5 cSt (such as Amsoils MTF, Texaco's MTL, Pennzoil's Synchromesh, GM and Chrysler's Synchromesh)
3. 75W85 Series-Type; 9.8 to 11.5 cSt ( Redline's MTL, RP's Synchromax LT, Nissan's MTL, Honda MTL, Castrol Syntorq LT)
4. 75W90 Series-Type; 12.8 to 14.5 cSt (Amsoil's MTG, Redline's MT-90).