How is MB going to know if you had run a tank or two of B20 or B5?
The big concern with running Biodiesel above 5% is crankcase oil dilution with post combustion injection to clean (regenerate) the dpf
If you run a tank of B20 just follow up the next tank with pure diesel and you'll be ok. If B20 is prevalent and you can't avoid it, reduce oil change intervals to 5,000 miles and you'll be ok.
I've got a 100k miles on my 3.0 MB engine and have used B20 about 1/3 of the time. Engine has been fairly reliable except for a bad turbo actuator which I replaced last month. Speaking of turbo actuators, they don't sell them in the USA and dealerships will want to sell you a 4,000 dollar turbo. You can get the actuator rebuilt for about 200 dollars overseas!
Don't get stuck because you are afraid of running B20 fuel. A little B20 occasionally won't hurt a thing but do not use extended oil intervals.