I replaced mine with the Ford "honey" fluid last Thursday. You can get it at a dealership, ask for part number XT-M5-QS. Be prepared to pay $20 per quart, unless you know someone there who can get it for $15. You'll need 3 quarts, though it may only hold a little more than 2.5 quarts.
I did notice smoother shifting immediately following the change, however, it has stiffened up a bit since then.
If you aren't a real aggressive driver then I wouldn't worry about about using the Mobile 1 synthetic.
