The recommended amount of oil is different between the early new edges and the newer ones. I think the recommendation changed in 2001 from 5 to 6. Ford also changed the recommended oil from 5w-30 to 5w-20. I'm not a mechanic but it has something to do with the way the oil flows back to the oil pan and that it might not flow back fast enough at high rpm. The arugments for 6 quarts were sufficiently strong enough that I now run 6 qts in my 2000 GT.
6...Go to walmart its cheaper...you save a **** load buying the 5qt jug...then a qt...or you could just buy two 5qts and pour in a qt out of the 2nd each time you change and when your out buy a 2nd 5qt again...that's what I do with mobil fully synthetic...do the math you save alot