i am pretty sure when i (or we) am talking about mainstream, i am referring towards shell/castrol/petronas etc.

the brands like amsoil/redline/motul/lucas you have mentioned are all considered boutique/non-mainstream brand, in which their price clearly reflects btw

And if you've bothered checking out my links earlier (carbibles.com btw, or you can even look up bobistheoilguy forums) you will see that not all amsoil products, especially their top of the line/flagship are API certified, and they have even put out fake certifications

slight note of bobistheoilguy btw: they have many "science/chemist" guys (lol!) that does UOA and results/feedback from torco has been pretty good mind you. else i wouldn't bother pouring something un certified with similar/same concerns as you are having

i mainly use liqui moly and only used torco for my car for that one time track day OCI. if you didn't know, liqui moly is API SL / ACEA certified. so isn't that good enough? or what other kinda certification you're looking at?

atm i am considering changing to liqui moly for the nissan sentra because my mother is the main driver of it and she doesn't high rev/drive fast much, which kinda make torco pointless to be honest. might as well have the extra protection/engine quietness that liqui moly provides.

for my own info's sake though, what kinda certification v kool has? the one they give you when you buy their elite/high end product? or the one from jpj? please elaborate
and lol surprised to see you mentioning track
however my reasons or even justifications as you may call it stands, think logically: why are we buying better engine oil? is it because it offers better protection and provides a smoother engine?

it's pretty much the same deal with lubegard.
the exact same things you said can also be applied to engine oil, why waste money on better engine oil for prevention? why not just change your oil at tighter interval? (4k km? 3k? or even 2k km? from my experience, shell lasts 2.5-4k km) That guarantees engine smoothness and cleanliness too. no point to buy more expensive oils right?

its just the same concept/logic. time to time i'll rev my car to red line just for the sheer fun/exhilaration of it, i don't think having more frequent OCI (be it engine oil/gear oil) would help much. and as i mentioned on my previous post, the gear engagement of my gearbox with and without lubegard is pretty obvious. it could be because of my very old gear box, or its because of my driving style having higher revs on gear shifts, i wouldn't know. however the differences is there, and i'd like to keep it.

hell, even lubegard's own statement claims a 50% increase in gear response which is 0.4 sec to 0.2 sec (lol minor almost unnoticeable difference, but it IS 50% LOL!) which may or may not be true, however for my case, it is even bigger difference especially on cold start putting my very first D or R engage after engine crank.

=edit=
and yes, while i've only been a lubegard user for maybe 1-2 years, definitely not going strong like your 8 years record. 1-2 years without any gearbox issue on my old 3 speed auto proton tranny thats been to track day once downshifting/1-2 speed gear limitation at 5-7k rpm... can consider *a bit* credible, right?

of course, like you said, at the end of the day, my money, my choice. and that's that la.
however we always can have healthy debates (hopefully not arguments) for knowledge sharing right?