Just to share, I had a similar symptom on my R32 GTR (77,000km) that lead to a transmission replacement.
I had a crunch in 4th, and eventually developed a crunch in 3rd at high rpms. I switched the fluid (nothing special) in hope that a fresh fluid would help eliminate the crunch but it just made it more prominent.
Anyways, I have yet to find a permanent solution to the synchro issues on the R32 transmission, and it seems like no one has ever found one. I have been reading that even the R33 boxes have similar issues.
I know I am going to have to replace the box again at some point in time.