Thats a good question, and octane was one of the subjects in a new Q/A column in this weeks Wall Street Journal.

Mercedes benz is very specific about how the car should be used on their premium cars if only medium grade gas is present, no more than 2/3 throttle, no full throttle, no more than 3K rpm, refill with premium as soon as possible.

Rental cars get rid of their cars at what fixed mileage? and if any probems were to develop from using the wrong gas the car would probably be long gone from their fleet.

Could the wrong fuel grade cause an issue where potential litigation forces rental fleets to stock two fuel grades?