The engine in your MB probably has a gasoline engine with higher compression ratio, and closer tolerances than run of the mill engines. Thus the engineers specify Premium Fuel for optimum performance. However, they realize that some owners will mistakenly use the wrong octane fuel and protect the engine with a knock sensor. Knock is pre-ignition. When a knock is sensed the timing is adjusted by the computer , but the knock already happened. Now your driving with less than optimal timing. Not a good thing.

I can't understand the financially logic that trades 10 or 20 cents per gallon in exchange for worse fuel economy, and risk of damage to a very expensive engine. This is a Mercedes , not a Chevy. Let's really go crazy and assume 25,000 miles per year at 20 mpg = 1250 gallons per year. Take that times .20 cents and you arrive at the princely sum of $250.00. Mid-grade saves you $125 per year!

When I read the words, "It runs fine", I want to ask, "Really, when was the last time you took the heads off to look?"

Steve - I follow the manufacturer's specifications for fuel and other fluids.