Its a combination of time and RPMs, which are the factors that lead to oil quality. Mileage alone won't account for an idling engine for instance. But yes, if you put in used oil the car won't know it.
It seems not all Fusions can be reset by the instructions in the owner’s manual. I have a 2010 Fusion SEL and I have to quickly hit “set-up” twice for the OIL LIFE to appear and only by hitting it twice will it remain on OIL LIFE so you can then hit “reset” to get it back to 100%