I would never change the oil at 15-22,000 kilometer intervals.
I use the FSS in this way:
At each major service (A or B) I reset the FSS. Then, each time it counts down 5000 kilometers (about 3K-miles) I change the oil and filter. That way, I use the "brain" of the FSS to help set intervals, but they are much shorter.
During summer highway use, I've seen about 7000-7500 kilometers (max 4500 miles) between changes. in hard winter driving, it works out to less.
And I use nothing but Mobil 1 and OE filters.
1998 C230 "Black Betty"