One gear like that suggests the sixth gear cluster is bad, prob synchronizer. Why this hits high gears is they are physically smaller and can take less punishment. 6th gear indicates possible abuse by a po. A fluid change is unlikely to help this but, you could always try it, save the old stuff and put it back if it doesn't help. Redljne mtl worked best on my last car. It's an easy job to drain and fill. Ford doesn't usually check the fluid beyond making sure some is in there.
If this is the only issue, you can test Ford and see what they say. It is in warranty. Or you could just drive around it with technique. As you go to pull sixth, you'll feel the beginning of the drag as the shifter engages the synchro, wait there for a second to allow the sychro to speed up and the gear will slip right in, grind free.
You may wish to install a shifter bracket or some other form of improving shifting precision. The stock shifter without help is imprecise and a bit flobbery.