I have been mostly doing exterior changes to my 05 STX and wanted to start looking at upgrades under the hood. As I began my search, most of the stuff I find is for the 4.6s or 5.4s. I am primarily looking to increase some mileage and efficiency. Any recommendations are welcome but for now I have the $$$ to do the light to moderate stuff.

Does anyone know what I can do to the 4.2 to help increase fuel mileage and/or efficiency? Its not that I'm complaining now, just planning ahead.