There has been at least 500 posts on the gasoline issue on various forums.
There are a lot of misunderstandings, misconceptions, rumors on the real value of premium gasoline over regular and/or mid grade.
Which one to choose???
Maybe my 25 years experience with a major petroleum products refiners can be of help.
1) Engine manufacturers: They base their recommendations solely on the compression ratio of their engines vs. grades of gasoline available in the market they sell their cars. Generally, manufacturers recommendations are on the (very) safe side. Why would they risk having engine problems (warranty, image) by specifying a ''good enough'' gasoline grade. So, they tend to be very conservative and generally would require a bit more octane their engine realy need.
2) Petroleum companies: They will try to sell the highest volume/proportion of the premium type. They make more money on premium than on regular gasoline. Typical marketing approach. The higher priced, the better, they say....
3) Consumer: many feel that buying the most expensive will garanty the best buy. A $6,000. Cartier watch is certainly percieved as being better than a $10.00 Timex watch ( In terms of keeping/giving the exact time of day, Timex is probably as good as a Cartier). As consumers tend to be influenced by 1) Engine manufacturers and 2) petroleum company marketing campains, They are left with the impression that premium gasoline must certainly be superior than regular grade gasoline...... Not necessarily true for most applications. And those impressions are transmitted from one generation to the next.
Thruth: Premium gasoline differs from regular gasoline ONLY in their ability to resit pre-ignition (pinging). If any given car can run satisfactorily with regular grade gasoline, filling up with premium grade is a total waste of money. No more power, no more MPG,
Exception: On some newer cars, (not on a 300E) total timing dwell is retarded on the onset of pre-ignition. Retarded timing reduces power.
Some (only some) premium gasoline could contain a bit more of fuel system cleaning additive, which is not always required.
They best way to find out which gasoline your car prefer is firsts to go to a mid-grade if you have been running with premium. Open your ears, listen carefully for pigning/pre-ignition noise on acceleration. If you ear nothing bad, try regular gasoline and listen. No pigning? regular gas is for you.
That's unless you still want to give your money away to the big oil companies. Your choice.