When you have a car with high compression or forced induction (Turbo) etc. you run the chance of detination. That is when the piston compresses the Air/Fuel mixture it may heat up enough to detonate before the spark plug ignites it. This causes large amounts of heat and can melt you pistons. Higher Octane Gas is not as volital, so it prevents detination(pinging, pre-ignition, knocking). Most all autos have knock sensors and the 300E will retard the timing untill it stops knocking if it can.. If you experience this let off the gas and add high octane ASAP. I have been running 90 Octane during the winter with no problem. The colder the intake the less chance of detination. I havn't tried 87 Octane nor 90 during the summer or a hot day..

It has been recommended to me to try 90 and if it knocks/pings to up the Octane. This can be risky business if you don't pay attention to the sound of your engine. The person who owned the car before me said he had used 87 Octane often??

In my Turbo Talon auto I use only 92 or higher. If i turned the boost up beyond 18 PSI I would have to use 105 Octane.

I feel safe with 90 in the MB...

P.S. The lower the altitude your at will also increase the need for high octane.