I've got a 2000 W202 Mercedes C240. It's got a V-shaped, six cylinder gasoline engine. On the fuel cap, it says RON 95 EN 228. And in the manual also it says "Use only Premium Unleaded petrol". I sometimes want to try RON 98 as it's imported from Europe and everybody praises it. But I don't know whether it would harm the engine. Does the manual mean at "at least" or it means what it says- "only, no lower, no higher"?
It means "at least". You will not cause any damage by using the higher octane fuel. The reason for the RON 95 rating in the first place, this is the lowest octane rating you can run before you might start hearing pinging or pre-ignition.