And for me a car should be designed to use the lowest octane generally available unless there is a good reason to need more. While a boosted engine that lacks direct injection may need to control pre-ignition (detonation), a well designed NA engine should not.

Historically, octane available today is quite hgh but far below 130 octane av gas. PON is a kludge. Back in the 60's gas was rated by the Research method because the nubers were the highest and high numbers sell. Motor method numbers were considerably lower but a better guage of resistance to detonation.

With the oil crisis the Fed decreed that henceforce gas stationy would display the average between research and motor method and it was found to be about 4ish points lower than research. So a 98 octane "premium" becane a 93, 93 octane regular became 89, and what Sunoco had been selling as an "economy" grade 91 became 87.

Lead poisons catalytic converters so TEL had to go. Meanwhile Amaco had been selling uleaded for years, just was a few cents per gallon more expensive but had many advocates.

Change was slow and by 1984 you could still buy both: leaded and un & mfrs had to use a special noddel for unleaded & make gas tanks that would not admit a leaded nozzle.

Meanwhile high altitudes make for lower peak chamber pressures so a car designed for 87 at sea level runs happily on 85 in Denver.

Meanwhile the main reason for high octane, giant V8 engines with 4+" (100 mm) bores, usually with wedge chambers and the plug over on the side (remember "angle plug" heads for SBCs ?) needed "premium gas" to have time to completely combust without forming a secondary flame front ("knock" is when two flame fronts collide in a cyl).

My Jeep has a 3.6 with about the same bore as the 3.2 Pentastar just winds a little higher and produces 290 hp on 87 PON. Just is about a decade and a half newer design.

So while MB ays 91 octane, which 91 octane (Europe has different ratings yet and why. At the power level (215hp) and small bore it should run well on 87.

So why (other than we know more now than then) ?

BTW I should now have more time than ever to pontificate.