2005 G35 Sedan with 6MT and 20,000 miles. Before it is fully warmed up ( 5+ miles in 45 degree F weather ) I sometimes have a problem with the 6 to 5 downshift grinding. It is like the synchro is not even there. There is no resistance to moving the lever from 6 to 5, it just goes right in but grinds. Has anyone else had this downshift problem? Has anyone experimented with other transmission fluids? I know on one previous manual transmission car Red Line MTL made a big improvement in the way shifts occurred.

does it ever grind from 4->5? probably is a synchro issue, at least you're under warranty.

my 05 does this also only 1 time a day, and only on the first downshift when cold. I thought it was me not putting the clutch completely in or something, but I've tried everything.
What I found that works is that if I hold off on 6th till its warm it wont happen...

anyone else?

The other thing that I noticed is that it happens more at 40mph or so than at 50+mph. I wanted to check on here because I read about other synchro problems, but not from downshifts and only when cool to cold. My problem started with the first downshift, but now it happens more frequently.

I have an '05 sedan and I started getting the cold grind going from 6th to 5th. I now have the cold weather whistle coming from the tranny for the first 10 minutes of driving. New tranny in my future? I have exactly 21K miles.

mine used to do it from 4-5 at low rpms. finally when i blew my 3rd gear they replaced the tranny and its very smooth now.

3rd-6th grinds for me, But only when you dont let the rpms drop more then 500 rpms its weird.

New Trans going in Later this month