As others have said, this should be fine in your Focus. I would point out though that Ford specifies 5w-20 meeting WSS-M2C945A. 0w-20s would be the wrong viscosity and would meet WSS-M2C947A instead.
While pretty unlikely, it would be possible for a dealer to be difficult on a warranty claim as the specified oil wasn't used. I have a Ford and stuck with 5w-20 through the warranty period for this reason. With not much to gain, why take the chance?
Others will chime in that Ford would have to "prove" the wrong oil caused whatever warranty problem they refused to warrant. This may be so, but do you want the delay and hassle this could involve?