Just for reference, I've been using 87 in my '87 560SEL for over a year now with no problems. When I first got the car I ran several tanks of premium before switching to compare performance and fuel economy. Without a dyno and/or a stop watch, I doubt anyone could notice a difference in performance. The fuel economy was marginally better (about 3-5%). Given the 15-20% difference in price between 87 octane and 92, my wallet comes out way ahead on 87 octane.
Several years ago, I tried experimenting by advancing timing on a '73 Plymouth until I was just below the detonation point with premium and compared this to running the same car at factory tune and regular. Since this engine had a somewhat lower compression ratio than the MB (8.5 vs 9.0 or 9.2) there was a more significant difference in fuel economy but STILL not enough to make up for the premium price.
Therefore the ONLY car that I run on premium is my '67 Dodge with 10:1 compression. Even so, I have to run the car slightly detuned especially since about 1/3 of the time the premium gas I end up with is definately NOT premium octane (ie the gas station cheats).
'87 560SEL 267K (177K on motor) Blue/Blue
'98 Buick LeSebre 60K (wife's car)
'56 Imperial Sedan 124K
'67 Dodge Monaco 130K (Sold)
'87 Chrysler 5th Ave 245K and going strong (sold)
'73 Plymouth Satillite 175K (sold)
'96 Chrysler LHS 80K (totaled)