I have a 91 300E 130K pre-multivalve engine. All said here in general holds true from my experience. I run 90 Octane and have not experienced mileage nor apparent performance degration. Now I wouldn't try 87 Octane but 90 seems to work in my car. Maybe if it was 0 F or below outside. I have not driven it in very hot weather being up north(MN). The hotter the intake the higher the risk of pinging/knock/pre-detonation. So I will check again in August.. This works in my MB and my area, but I would not generalize for all cars. I have an Eagle Talon Turbo running 18PSI of boost and I would never use less than 92 Octane, this would melt my pistons.

Higher Octane can be a waste of money but I would follow manufactors recommendation. My MB model appears to have good knock protection so it will retard the timing as much as possible. This should show up in your gas milaege, so if your have a good baseline of mileage at X ambient temp and 92-93 Octane then you can drop the Octane to 90 and see if your mileage suffers. Conversly in a car that recommends 87 Octane if your mileage doesn't go up using 90-92 Octane then your wasting money.

P.S. Ethonal actually helps to prevent pre-detonation. All gas here is 10% Ethonal.