Engine cleaning products make me verrrrry nervous. Dislodged sludge can be a ton more damaging than a little that's harmlessly built up on engine components. I wouldn't risk it. If sludge is a real problem, a rebuild might be in order. (At least pulling the valve covers and seeing the state of the top). I haven't heard of sludge issues with windstar engines anyway. I have a camry with the 1mzfe that's known for sludge and even that appears fine to me. [i think it's really just a problem for people who don't change their oil but that's a discussion for another day]