The FSS can display in days or miles... if it sits for a long time - it will most likely display days, otherwise it is miles.

Our cars with synthetic can go roughly ~7,000 miles until an oil change.. If you change it every 5k or so you're just wasting your own money, but to each their own.

It'll flash the wrench (or two wrenches, depends) when you start your car up if it is time for service.