Resetting your FSS is in the owner's manual.
I set mine to 15,000 kilometer (just shy of 10K-miles) countdown. I then do an oil/filter when it has counted down to 10,000, and then again at 5,000. At 0, I do either an A or B service (alternating generally) and reset the FSS.
The A service can be done by anyone who can change their own oil.
On the 2001 models that have included service, I wouldn't reset the FSS myself, as it means fewer "paid for" services by MB. I would:
Change my oil/filter myself every 5,000 kilometer countdown. Have MB do the A and B services at their cost when FSS reaces zero. They will then reset the FSS.
I would NOT leave the oil (even Mobil 1) in for the entire interval of the FSS unless I was leasing the car and intended on returning it. Since I like to keep my cars for 400K or more, I would never dream of such long oil/filter intervals.
One minor problem with DIY oil changes on the W203 cars with the M112 V-6 is the lack of a dipstick for checking the oil while filling from dry. I know MB sells one, and it would be interesting to know the price.
1998 C230 "Black Betty"