My opinions: we all have one - Ha.

The argument that if you drive a Mercedes you can afford an additional $300a year in preminum fuel does not hold water with me. An '87 300E is a $2000 car at best. I doubt anyone would consider running one of these very hard, and expecting it to live long, no matter what fuel it had.

Mercedes indeed goes a bit overboard in their maintenance and fuel recommendations. It's to their benefit; these cars are driven in horrendous conditions all over the world, and MB wants to push owners to the very best maintenance, to help them retain their reputation for reliability.

The resistor chip in US models retards the overall timing quite a bit, obviously don't mess with it if you are going to run less than preminum.

The '87 103 has no electronic knock sensor, and the timing advance is only adjustable by changing the timing resistor. And the US resistor is pretty retardant already.

The 103 has aluminum heads with 9/1 compression. That's pretty low.

You can definitely hear knock and ping in the 103. I heard it once in over 10 years of running 87 octane fuel. I wound it to 5000 rpm at full throttle on a 100-degree day, and picked up just a bit of ping as it went past 5000.
Otherwise, zero indication of any issues running 87 octane in hot Dallas summers with the AC runining wide open.
Continued ping and knock will show up quite obviously in most engines by checking the spark plugs. I can't knowledgably say this is true of the 103 because I check the plugs at least every 10k miles, and have seen no evidence of spark knock damage for the last 125k miles.
Maybe I have incipient knock and don't know it, but......

For me, running 87 octane has seemed to pay off. over 8000 gallons used, at an approximate 30 cents savings = $2400 saved. Enough to overhaul a 103 engine if it's getting damaged and falls apart at a mere quarter-million miles.

Just my 2 cents.

DG