The Mercedes FSS system maximums are set to 10,000 miles or two years, whichever comes first. BMW's average about 10K in their system. It's not 20K miles. Mercedes owners were using synthetic oils long before the extended oil change intervals came about.
The Flexible Service System (FSS) has a different name in some other markets.
If anyone doubts the benefits of synthetic oils, they need only know that Toyota caused oil to gel in their V6's by increasing the oil temps in the heads (smaller passages) to reduce emissions. Petroleum based oils gelled if not changed frequently. Synthetic oils did not. Toyota has sinced opened up the passages to fix the problem.