Turns out car grinds in 5th gear.. noice...When test driving the car I don't know how but never came across it. What would be the best option to rebuild the synchro, or get á whole new tranny?

 

Used tranny will be cheaper. Rebuilding will require a large gear puller and a press.

 

I heard the cd009 is good? Does that work for the 07 08 hr 350's? And where can I source a used one?

 

CD009 is not suitable for the VQ35HR. HR engine uses JK40 (before 7/7) or JK41 (after 7/7).

 

You can use any Trans from the 370z as well as the JK in the 350z HRs. This is not the same as a CD009/CD00A these are for VQ35DEs only. not the VQ35HR you have.
That being said is it cold where you are right now? Is it only 5th? Does it only grind in the cold? If so that's normal with non Nissan fluid. drain it add OEM
By cold I mean like 45 deg and under.

 

Drive till it fails , could be 70k more miles

 

Change fluid.... might be that simple

 

Different trans, different situation, so take as anecdotal. I had notchy shifting experiences with my CD009 after replacing OEM fluid with Motul Gear300 (75W-90). This experience was only prevalent in the mornings (1st start of the day). After the trans had some time to warm up from a few miles of driving, the shifts would smooth out and behave nominally. After some time with this, I decided to give Redline MT85 a go and haven't looked back since. Note: I was planning to take my Z out for occasional track days, hence the Motul fluid. But for a more DD centric approach, I believe Redline's MT85 is the way to go. It's common practice to replace OEM fluid with Redline MT85 which was specifically formulated with Zs and Gs in mind.

It's a very simple process to drain and fill your gearbox. It's also inexpensive. As others have suggested, I would try replacing fluid 1st and put the Z through it's paces. Maybe that's all it will take, maybe not. Still worth a shot IMO.
Good Luck!
-Icer