Hello, i have an issue with 3rd and 4th gear grind on my CD009 350z Trans

When i initially cold start my car, and take off, 3rd and 4th doesn't grind as much. And if i shift extra slow, it doesn't grind.
But once the engine warms up, it grinds like CRAZY. Like, I'm talking about KRRRRRRRRR. It sounds BAD. It grinds even HARDER when downshifting from 4th to 3rd, or from 5th to 4th.
Now, the gears DEFINITELY hold power. When i go into 3rd, it locks HARD, and doesn't pop out by itself.
All other gears are perfectly good, no grinds, goes in like butter. But what puzzles me is 3rd and 4th doesn't grind as bad when the car is cold. Could it be Oil? Slave cylinder? Or even worse.. Synchros?
Also, it has a brand new stage 4, 6 puck Clutch from ACT
Would love to hear any prior experience / insight on my issue.

Thanks!

P.S. I've attached a little video of the grinding. Eeek! it sounds GNARLY.

 

Welcome- how many miles and on your Z and what kind of tranny fluid are you using?

 

Thanks! And as far as the oil, I am not sure what kind of oil I'm using, as I haven't changed it since last owner. Mileage is 80k miles

 

Since you're not certain of what product is in your tranny and it's past the 60K change point, you might try to change it out. I prefer Redline MT85 as it's a GL-4 rated lubricant with viscosity rated for what Nissan recommends. It was engineered with enough friction to allow the syncros to work properly.

 

You can try changing fluid but most likely your third and fourth gear synchros are junk.