Driving style or factory defect the likely cause. Can't blame a GL5 for a 'busted 6-speed'.
I put over 250k miles on a 5-speed using GL5 gear oil. When I cracked the case, the guts were spotless. Shifted pretty good too. No damage to any bronze components.
The real issue is most GL5's have LSD additives which in some transmissions can cause shift headaches. If you want to use GL5 in a manual, look for a non-lsd gl5 gear oil.
For any gearbox, I would always prefer a GL5 over a GL4. Transfer cases tend to be undersumped, overstressed, and undermaintained. So, I would think that the added protection is a plus.
If shift quality isn't as good as you want, then use the GL4.