Yes the grind/lockout is a trans issue.
The syncro's can't match speed fast enough at high RPM. Using a lower viscosity fluid helps.
I run 5w-30 motor oil and shift without issue @8,000RPM.
The thinner fluid allows the syncro's to catch up faster, less resistance.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.