Just wanted to share a little personal experience with my '04 Xfire (which continues to be a great car, by the way!).

As my car will soon have been in existence for 4 years - and, even though it only has about 16.5K miles on it - I decided to change the transmission and differential fluids.

If you haven't done this, then I strongly recommend it - both were terribly dirty (no performance issues) and in bad need of change! The magnetic plug out of the transmission looked like Gabby Hays' beard, so I was pleased to see it was doing its job!

Replaced transmission fluid with synthetic M&B Slick Shift and the diff fluid with Mobil 1 gear fluid (75W-90).