My first experience with the FSS system was in 1998 when I got a 1998 E320. Mercedes makes it sound like FSS is a sophisiticated sensor system that actually analyzes the oil and determines when a change is needed. Instead, I think FSS is nothing more than a computer program that measures engine operating factors, driving styles, driving time and distance. Based on these inputs the program calculates when the oil should be changed using a predetermined Mercedes guideline.
The FSS system usually tells me changes are due about every 11,000 to 12,000 miles which is about 1 year for my cars. In both the 1998 E320 and the 2001 E55 I now drive, I change the oil at about 5,000 or 6,000 miles using Mobil 1. After these changes, the remaining mileage until the next oil change doesn't increase at all. This tells me FSS isn't actually testing the oil quality. I bet if you reset FSS and replaced the oil with old crankcase oil it still would tell you the next oil change was due in about 10,000 to 12,000 miles!