I recently bought a 05 350z and it's my first ever manual. However I'm having some problems with downshifting. I can shift up pretty okay but downshifting is a lot harder. Very often I get stuck trying to downshift from 3rd to 2nd having to use both of my hands to put it in gear. This could be me being too weak as I was born with weaker muscles than normal and I'm trying to work out more to compensate. But is there any way to make shifting more buttery smooth? I've already changed the transmission and gear oil and it's still hard to shift down. Any recommendations would be helpful!

 

If clutch and transmission are known good (which a new trans and clutch should have addressed) and shifter alignment is good…

Rev matching and learning to heel-toe downshift will help immensely.

 

THIS^^ Rev-matching and learning to properly heel-toe downshift will also reduce syncro wear in your tranny. Plus, it's a skill that transfers to any other manual you buy in your life.

 

From a mechanical standpoint, consider upgrading the clutch pedal to the RJM Performance unit. Also, consider bleeding the clutch hydraulics or replacing the master/slave cylinder if leaks are discovered. I doubt the hydraulics need attention since up-shifting doesn't pose any issues. But you will get a noticeable quality of life improvement from upgrading the clutch pedal. Otherwise, what dkmura and Mic have recommended (rev-matching) is your best bet for safely downshifting. Failure to rev match will cause premature wear on your clutch and just make driving very uncomfortable. I'm sure there are plenty of YT vids out there showing what Heel-Toe shifting is, it's benefits, and how to do it.
Good Luck!
-Icer