A few years ago I consumption- and performance-tested 95 RON and 98 RON over a 6-month period on both my M130 280SE and M110 280SE.
Both cars usually gave similar consumption figures on identical fuel. The extra cost per litre of 98 over 95 was bettered by a percent or five (I can't locate the record just now) lower consumption in km per tankful; i.e. 98 proved more economical over the tankful, performance was notably better, and the cars seemed to maintain better tune between services (especially notable being my twin-Zenith Finny with rebuilt carbs). Win-win-win (the first winner being, of course, the oil companies and government coffers).
In just about any Benz, I reckon 98 is best for car and pocket.
I'm not into additives in the dailies, but my 'ancient' Sunday drivers get Shell's version of Flashlube with each fill -- just for the extra feelgood vibes.
Once every couple of months I add 0.5% (half litre per full tank) synthetic two-stroke oil (Stihl, made, I think, by Shell). Seems to keep things really clean, makes no smoke or ash.