Just to follow-up on this old thread, and input 2017 conditions.
Today in Texas premium gas is 60-70 cents more than regular, or about 30% higher!!!
This is a whole new ballgame for the regular vs premium discussion.

Not factoring in increased fuel MPG, that's an average savings for me of about $400 a year!

I drive a '91 300SE with the M103 engine.
I have run strictly 87 octane for the last 20 years and 200,000+ miles, with zero observed fuel-related problems. I have always gotten about the same avg MPG, whether using premium the first 7 years, or regular the last 20 years.

The M103 is faaar from a high performance engine, making only 59 HP per liter and having a low compression ratio of only 9 to 1. Having an aluminum head makes that low number even less susceptible to any octane knock.
My old car is heavy, and it gets hot in Texas; but I have never heard any hint of octane knock, and she's running great pushing towards 275k miles.

That's just my experience of course - yours may differ.
But I've saved at least $6-7k over the years buying regular - that's serious money.

So, my experience prompts me to recommend regular on these old engines,
and and save about what the car's worth every couple of years.

Cheers,
DG