originally, your car was made withe mercon iii. now, i switched to mercon v in my 93 when i had it, but i made sure the tranny was completely drained of all the old stuff first...including the torque converter. i had no issues once switched to mercon v. little heads up though....ford is discontinuing mercon iii permanently(inside info), so eventually, you'll have to switch to mercon v anyways if you stick with motorcraft fluids.
2003 Ford Expedition: 5.4L, harley supercharger, 2.93 upper pulley, 4lb lower pulley, c&l plenum, jlp cai, LFP SBTB, FTSP, Magnaflow 3" catback, Eclipse AVN-5510, white face gauges, cobalt c2 boost gauge, ddm hid's, 379 rwhp, 465 rwtq sold 11/3/12
2009 Ford Flex Limited AWD