I have only put, maybe, 2k miles on my '04 coupe since an oil change almost two years ago, and I've been getting the FSS single wrench display for a couple of months now - and, it's all just been weekend downtown cruises and easy freeway driving for me. I'm not excited yet, as I've only rolled an additional 250 miles (the display sez so), and have it on my "to do" list during my vacation next month. It's full of Mobile One, and it will be OK.

Based on that, my GUESS is that it keeps track of mileage AND time, and lets you go about 20 months or so (roughly?) OR 7K, which ever rolls over first. OR, maybe it simply quantizes time and mileage, and adds them to get a sum that it uses to calculate when service is due - that's a stretch, but possible. That's the only way I can make sense of it, because mine has NEVER been driven hard (by me), and is nowhere near halfway to 7k miles since my last oil change.

FWIW, I have read that it's a good idea to change the oil every couple of years regardless of the mileage, to guard against breakdown over time and impurities. I guess MB agrees.