Costco

I would recommend using Honda ATF-Z1 only for automatics. A voided warranty is only one incorrect fluid drain/refill away.

and if it matters, I went on the cheap with my old Integra 5-speed and got Pennzoil Synchromesh (which is a similar formula to the GM Synchromesh, but the Pennzoil SM does not contain the Friction Modifiers for the TL's LSD 6MT) and it made my car shift like butter. I'm sure the GM stuff will work wonders.