I have the German adjustable unit on the Euro 190E-2.3-16 in the family, and the main reason for the feature was that unleaded fuel in Europe was not alway available with a high octane rating. So, you see one spot on the adjustable feature labelled "N" for "Normal" or "Regular" octane, and one "S" for "Super" or "Premium" high octane fuel. And, one position is available with no resistor or an infinite value for resistance. I always run premium and have set my resistor value to the infinite setting. There is a discernable difference in performance with this car, but not at idle as a previous post notes.
I am not sure about the rest of the timing advance vs. resistor values noted, as they seem to be a bit random, which can be programmed into the EZL computer I suppose, but it would be a little more complex with no apparent payoff. Benzmac or another technical expert might pipe in here and let us know. On a 190E 2.3-16 anyway.
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)