Unless the filter manufacturer approves a specific filter for a specific car, why take a chance while your car is under warranty. While it's really unlikely, if you have an engine problem under warranty and you've used an unapproved oil filter, it's one more excuse for the manufacturer to be difficult.
Ford in particular is really fussy about filters. If you have any complaint about engine noise, the very first question they will likely ask is what filter you're using. If it's not a Motorcraft, they will suggest that is the problem.
If you use an unapproved filter and an engine problem develops, do you really think Purolator or Motorcraft will stand behind you?