I'm pretty sure the original car ( 91 300E M103 ) has knock sensors so it would pull timing back as needed.

More data points:

I'm pretty sure that it is a USA requirement that a car has the capability to operate on regular and not damage its self.

Most / all modern engines ( mid 80's and up ) have computer controls and knock sensors that pull ignition timing back as needed.

Some engines loose lots of timing so performance and fuel economy could suffer more that is saved by using low grade fuel.

My 97 SL320 ( 3.2 L M104 inline 6 ) needs premium fuel otherwise performance suffers too much. 4,000 lb / 3.2 L = 1,250 LB / L

My 97 C280 ( 2.8 L M104 inline 6 ) happily runs on mid grade, due to it being a 1,000 lb lighter car and more motor per Lb. 3,000 lb / 2.8 L = 1,071 LB / L