I read with interest, many postings recently about the pro's/con's of running lower octane fuel. My '89 300E has always idled a bit rough despite being well tuned. I have always run Mobil 91 Octane.
Another quirk which I have not seen mentioned in previous postings is the tendency for the car to almost always require two efforts to start except when very cold (winter). The dealer says this is normal???
Due to the ever increasing cost of gas, I decided to try a tank of 89 octane and am very pleased that the car runs as well as before and the idle is smoother and it now starts on the first try everytime. There is absolutely no pinging or any other indication that anything bad is going on..