Installed the stock/factory heat range (they ARE available in at least one step colder as well
) NGK Ruthenium spark plugs.
The factory plugs looked really good (almost white ceramic electrode surround, no excessive soot, and only slightly rounded center electrode) @ 18K miles, except that the gap was a little more than the .028 they started at from Motorcraft/NGK.
The idle at least seems a bit smoother than before, but will have to wait for a long trip to see if there is any actual mileage improvement.
I will move to colder plugs once I have a tune in the ECM.