When I was buying my Focus, I asked my sales rep at the dealer how the oil change notification system works, since I was asking about doing my own and what effect that might have on the warranty (never owned a new car before). He told me that it actually senses the viscosity of the oil, and when it reaches a certain point of degradation, the system indicates the need for a change. I don't know if this is just a marketing fib meant to make it sound impressive, but if it's true, that's pretty slick.
He also encouraged me to just rely on the notification, but said that if it put my mind at ease to do changes at a certain interval that I prefer, to go for it. Obviously, it's my car, so I can as I please, but he seemed to have a lot of faith in the system. I know it's his job to make the sale, but he impressed me with how he worked through the deal with me, so I put a decent amount of stock into his suggestions.