I dont know if you would be able to notice a difference. TBh I have GL-5 in my sm465 but i can assure it was a far better fluid than the stuff I pulled out. The stuff that came out separated to 2 distinct fluids, but one wasnt water......
It made it a lot quieter and seemingly better but I dont drive at speed or engage the 2/3 synchros (3 being the problem child I think?) , so I wouldnt be the representative person on that. I just remember reading here over a year ago that the metals are properly protected, brass and bronze that is, with gl-4 and GL-5 is more aggressive. If that means chemically or physically Im unsure. I would assume the yellow metals being softer than the other metals around them and most gearboxes these days not using brass or bronze to my kwowledge, the formulation just doesnt account for them. I recall using my owners manual for my 1980 to look and I do believe it said gl-4.
I just checked my 1980 owners manual, it says 80w-90 GL-5 for manual trans, no special notes. it says if in canada, use 80w GL-5
also it gave the otpion for a non multigrade GL-5 before the 80w90 but I forgot what it was. something with just one number or something.
Anyway, so I guess i am unsure.
@bucket you are knowledgeable on Gear Lube can you learn us the answwer
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I looked into a 1986 FSM just to due dilligance(since my trans isnt a 1980 in the truck) and the same data... SORTA. It indicates to use a Dexron II(cant buy that anymore, so unless you want some used Dex II from my 1990 oldsmobile (well used) then DexIII would be the only available similar fluid that "retroactively" works in its place.
I would suspect this is for economy and shifting ease, maybe? I dont know how I feel about using an ATF in a SM465.
But it looks like the NP205 has this change too... curious about how good it is for them..
Anyway. I am unsure now. Mt1 additive is something I found on a quick search, something about avoiding GL5 if has limited slip additives, and something about synthetics... but it seems there is a consensus that either old transmission oriented products or ones that use MT1 additive (for the brass and stuff) is the best route, and that means you could have GL4 or Gl5. But I just literally googled that in like 3 minutes. So.