German cars are all known for high octane habits, and I guess MB hadn't gotten over the "normal" 96 or 98 otcane, high lead fuel typical of Germany up to the mid 90's, when the forests all started to croak for air pollution. Nasty.

Use the premium -- detonation at high speed is not audible, so you will get lower performance and burned valves/melted pistons. The M103 does NOT have a knock sensor, and will NOT accomodate lower octane nor adjust timing for it except for running richer to compensate for the excess O2 from poor combustion when it knocks.

The result of running lower octane fuel is poor performance, low milage, and possible engine damage. The milage gain alone will probably pay for the difference in price.

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!