I heard that, in the middle of 2002, they started installing a 6-quart oil pan, but before that it was always a 5-quart oil pan for the late-model Stangs. That's why my owner's manual says "6 quarts".
Your best bet is to ask persons who own a Stang of the same year as yours.
And you can't always rely on the dipstick, because some dipsticks are inaccurate, or so I've heard.
And don't put in 6 quarts if the capacity is 5 quarts, because it's bad to overfill it.