Nearly all electronically controlled engines these days have a knock sensor, which allows the ECU to retard the ingition timing and/or enrich the mixture to prevent knock. The engine will run fine, but lack power and get worse milage on 87 octane instead of 93.

However, on older Benz (and some other engines) that lack a knock sensor (on the Benz, KE-jet never has a knock sensor), running lower than required octane fuel will result in engine damage, usually from detonation at hig rpm/load that is difficult or impossible to hear.

Detonation and pre-ignition can do nice things like burn valves, flame cut piston rings, burn out cylinder wall, and burn holes in pistons. All are expensive!

With new cars, all you will get is a performance/milage hit, and reduced catalyst life.

I assume that most manufactuers will not honor the warrenty for rental cars if the rental agency routinely runs them on the wrong grade of fuel.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!