The manual for the 2005 GTP says use Dexron III type H, but the manual for the 2007 Base says use Dexron VI.

Napa has both Dexron III and Dexron VI, so I'll probably go with the Dexron III for the 2005 GTP. It's Napa brand, but it has to be better than the brown stuff that's in there now.