Long time manual trnmasssion owner here. I have used GL5 once a manual gear box. Never again.
GL5 gear oils are too slippery and your shifting will suck. I don't care if Toyota says you can use it. The MT1 spec on the bottle means nothing either.
It will feel more slippery which you may like but don't expect to change gears without the very occasional crunch.
If you have money to burn, get the Miata owners favorite. It is a true PAO German fluid. Read some reviews and why it gets a 5/5 by hundreds. I put in a VW GTI I once owned.
Actually I'm using it (really Castrol Syntrans Multivehicle alias Ford BO alias BOT 328/130M) on my MT transaxle no-hypoid with +60% torque. Previous was Motul 75w90 GL4/GL5.
Motul 75w90 GL4/GL5
shifting: very smooth warm and little hard cold first miles. Resume good for so thick oil.
formulation: mainly fake synthetic + PAO. Downgraded GL5 pkg.
Castrol SMV 75w90 GL4
shifting: very smooth warm and seems better cold than Motul. Obviously PAO helps and general visco is lower, but pkg too.
formulation: mainly PAO. Classic ZDDP pkg.
Same oil, wrong link. NS version is NS version.
Designed for transmissions and transaxles - helps to slow synchros for easier shifting across a broad temperature range
www.redlineoil.com
Contains extreme pressure additives like our 75W140 GL-5 oil, but lacks friction modifiers to balance slipperiness
www.redlineoil.com
.
Yes, GL5 NS will work, all there will work better or worse.
Question is commercial advise vs chemical/test reallity.
75w90 GL5 NS has, as most GL4+ and GL5, sulfurized EP additive polysulfide, it offers great load protection, for specific conditions and gears design, but it's really aggresive for typical MT alloys, this is solved with inhibitors as additive, then it passes well known cooper test.
Reallity
Issue 1: It's not the real chemical behaviour after x miles with oil properties and additive depletion.
Issue 2: That commercial cooper test is not full guarantee about chemical corrosion. There are corrosion tests from mass % loss that show as common EP additives like polysulfides and specially pentasulfides (x3,7 damage vs polysulfide) attack these metal alloys inclusive when you see commercial advise "fully compatible and cooper test passed 1a".