If your car had a factory installed Euro emissions system, you will have a small black, 1.5" or so diameter, plastic knob on the driver's side front fender with green lettering on it. It is an ignition timing selector and you set it according to the octane of the fuel you buy. If the car came from Germany with the factory installed system you will have a little decal under filler flap, and possibly one in the glove box, that says "Super Bleifrei" which is lead free high test. Which is 91 octane to us. If you use a higher octane you can set the knob to the "no resistor" position and get a few more hp. To find that you have to take the knob all the way off and look under the cap for how the pins are arranged. On the outside you will see "N" and "S" for "Regular (Normal auf Deutsch)" and "Premium (Super auf Deutsch)" and a bunch of other tick marks for in between octane ratings, as well as one spot with nothing. The nothing position, I believe is the no resistor position, but check it out yourself - the scheme is visible when you look inside.
If you have a Euro car that was modified over here to put an emissions system on it the car needs 91 octane or better. The octane rating is based on the compression ratio and almost all Euro models had 10:1 or so compression ratios. To run on a low octane and not knock cars use compression ratios around 8:1 or so.
Enjoy the car and good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)