The 103 engine does not have a knock sensor; so low octane gas can definitely be a problem - if it causes ping and detonation.
Susceptability to ping depends on the overall condition and tune of the 103 engine; and, as stated above, the ambient temp.

That being said, I always ran 93 octane in my '91 300SE. Once I tried 87, and sure enough, it pinged. But, later I discovered my EGR valve was bad.
after replacing the EGR, I have rfecently tried 87 octane, and no pinging whatsoever, yet. It's about 80 degrees here in Dallas, so, when it hits 101 degrees, I may find I need to run higher octane. Before, it never concerned me to pay 15 cents more per gallon to pamper a $56K car. Now, paying 50 cents more per gallon in my $4k car is more painful.

Anybody else have any comments on the M103 and octane requirements?

DG