f im not mistaken there was a year when MB recommended 87 for the m103 then changed the engine a little and recommended 91....

also there is a resistor that retards the timing to compendate for cheap fuel...mine is absent. Normal (US) is a 6 degree retard. Europe has an adjustible resistor..while US model have a fixed one...I dont know what this will play in running mid-grade 89 or lower 87 grade fuel).

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