My 1996 Integra DC4 GSI has had a crunch from 2nd to 3rd for a while now when I shift above 3.5k RPM, or shift too quickly. But today I noticed while I was driving at around 1am when the temperature was around 6º C (Very cold!) that the crunching was gone, 2nd to 3rd gear shift was smooth at any rev change, even 7000. But after a few minutes of hard driving, the crunching began again.

The car had not been driven since around 5pm so it had been sitting for around 8 hours and the engine/gearbox would have been very cold.

My question is, if I replace the gearbox oil with a thicker oil could it possibly stop the crunching all together? Thoughts and opinions on what I can do, best oil to use, etc.

Thanks