Ive always heard we had ngk iridiums stock but havent checked to ever confirm it, the motorcraft replacement at most autostores is a double platinum, always ran ngk in my integra and they are a great plug but its gonna be close to $30 in plugs for iridiums.
All the focus guys running the same motors as these trucks recommend swapping for ngk coppers or autolite A104's, A103's(heatrange cooler) gapped at .055 Will they last 80,000 miles like an iridium might, no, but you arent gaining any enhanced performance by continuing with iridium except for potentially longer service periods.
duratec sparkplug sticky from focusfanatics if your interested in reading the pages of debate.
http://www.focusfanatics.com/forum/showthread.php?t=251453