I agree with Mike, don't fret too much over the specified requirements. Use what works for your car. If the engine starts pinging under low rpm/heavy throttle, switch to a higher octane. Unless your engine has a knock sensor, there is no benefit to running high octane except to avoid engine pinging.
Technically, octane requirement depends on a number requirements....compression ratio, ignition timing, and how badly the combustion chamber has deposits (resulting in hot spots and increase in compression ratio).
My manual specifes 91 octane. Locally I have available the 87, 89, ad 93 octanes. I use 89 without any problem.
I have a question to others....Is it possible my engine is pinging but the engine has so well built to suppress sound that I can't hear it?
95 E320 Cabriolet, 131K