For the sake of gathering info on this topic, here's a link to some of the most thorough info from the BITOG forum with all the tested and dedicated manual transmission fluids grouped by kinematic viscosity at 100°C or the rough average operating temperature. In case you want to follow me down the rabbit hole.
As I understand it,75w-90 GL-4 fluid was originally recommended for these manual trannys but that was later revised to 75w-85. However all these dedicated MTF/MTL options didn't exist back then, in 1993 in my case.
So, here's what I know at this point, I think:
- 75w-90 is ~14.5cSt, original spec
- 75w-85 is ~12cSt, revised spec
- There has been a trend towards lower KV fluids over the last 20 years, improve, shifting, better flow in cold temps.
- The advances in additive packages makes lower KV fluids possible without sacrificing high temp cushion and lubricity
- API GL-4 is a necessity, not GL-5 or 4+ or whatever else, especially in an older trans.
So it seems to me, going by the info I've gathered, something in the 10cSt is probably where I want to be, slightly lighter weight than the 14.5cSt 75w-90 originally recommended and close to the 75w-85 revision. If I can find it, which is proving a little difficult. ATF is in the 6-7cSt range and seems too thin in my case but I can understand it may be great in a freshly rebuilt MT. If that's what a builder recommended then definitely go with it.
Valvoline Synchromesh is the ONLY GL-4 MTF option I've found in my town and it's currently $9/qt at O'Reilly's. I cannot get my hands on a standard 75w-90 GL-4 fluid locally.
At ~9.2-9.4 cSt the Valvoline Synchromesh is a touch lighter than I'd like but it's available locally and relatively well priced. So I'll give it a go and report our experience. The good thing is this old Pathfinder isn't yet a *needed daily driver so it could sit for a bit if it doesn't work well and I still need to special order Redline MT-90 at $20/qt.