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

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

Naw, absolutely no knock sensors on the '91. No way for the engine to know if it's knocking and pinging. No manipulation of ignition timing other than the hard mapping in the computer for speed, load, temp, etc. The only feedback system is the O2 sensor which manipulates the EHA fuel valve to maintain proper exhaust gas rich/lean mixture. Pretty crude old CIS system.

DG

Knock sensors first appeared with the 3.0 liter 104.980 engine.

Weird, I've never seen more than ~20-25c difference between 87 and 91 octane.

Dallas seems to have the greatest delta in prices: yesterday at the Shell station:
87 octane: $2.20
93 octane: $2.90

Over $60 to fill the 2015 ML350, which I am still afraid to use regular in, what with direct injection and 12/1 compression. Of course the GM/Ford/japanese cars are getting more horsepower on similar engines using 87 octane.
Ya, you vill use der premium, and you vill like it!

DG

It has been an average of 25-30¢ between grades here for the past couple of years or so. Means 50-60¢/gallon between 87 and 93 octane. I run premium in the Honda since it returns an easy 5-6mpg better fuel economy (~100 miles per tank), despite the car being designed for "regular" 87 octane fuel. The cost is a wash between the two, but the car runs better and I can go longer between fillups with the high-test.

The SL gets 93 octane on principle. It will run on 87, but once it gets hot, it starts pinging which doesn't do it any good.

DI engines are far more tolerant of low octane than port injection.

I recently switched to regular for my 95 E320. Here's why:

https://www.cstatic-images.com/stock...4451470707.jpg

The old arguments why we need to stick with premium don't apply anymore for cars with knock sensors.

I poked around further to find out why premium is so much more expensive. There's no refining or other technical reason; the gasoline manufacturers simply believe the premium buying crowd are driving upscale vehicles, and they don't mind paying extra for their premium fuel. They're gouging the supposedly well-to-do folk.

There must be one gazzillion threads discussing the basis for this subject.
M.B. set the timing of the M103's depending on the market and quality of the Gasoline.
You need to look in the owners manual for your particular vehicle and it will state the Octane to be used.

The famous R16 resistor sets the timing and hence the Octane rating to be used. Just do a search for R16 resistor.
here's an image of the resistor:
https://farm5.staticflickr.com/4073/...be27e863_b.jpg

Some markets had variable fuel so MB included a resistor wheel - you could set the timing yourself depending on the available fuel octane rating.
here's an image of the wheel.
https://farm1.staticflickr.com/199/4....jpg?zz=1

my m104 will use regular,but if I'm doing mountain driving in summer,I use premium,my plugs last longer.But when winter hits regular is fine.