Not sure about 2009 Fusions, but for the 2010s, the answer is on pages 22-23 of the owners manual:
1. Press and release SETUP to display “OIL LIFE XXX% HOLD RESET = NEW”.
2. Press and hold RESET for two seconds and release. Oil life is set to 100% and “OIL LIFE SET TO 100%” is displayed.
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%