The optimum fuel for any given engine is the lowest octane that will not knock or ping under the most severe conditions encountered.

That's all the higher octane rated fuel does for you, resist ping/knock, i.e. detonation. If you want to be cautious, try this approach: let your fuel level get fairly low and put half a tank of the next lower octane rating in the tank. If it knocks or pings, fill it with the grade you were using as soon as possible. If it doesn't, continue on with another half tank until you're confident that the grade is safe for your engine, i.e. it won't knock.

Do it all over again with the next lower octane rating, until you find your engine's requirements. Personally, I run my '87 260E on regular even though the MB recommendation is premium. Zero knock.

Cars with turbo or superchargers and/or knock sensors are a bit different. They are essentially "adjustable" engines and will react differently. You also may find different requirements for summer & winter, or carying heavy loads, or mountain driving. And so on.

Advantages to using lower octane ratings are fewer deposite on plugs & valves from less anti-knock additives in the gas. The only advantage of higher octane rating is more knock resistance. More than your engine needs is useless and a waste of $, as you've discovered.

Just MNSHO,

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Norm in NJ
Next oil change at 230,000miles