Most factory engines are specified for use with 87 octane. A few luxury and car engines require or suggest 89 octane.
I don't think any factory engines require more than 89. I'll guess that's why it's the default for places selling only one octane.
Most modern engines have knock detectors and will run safely on any octane. But your performance will degrade if you use lower octance than expected.
Obviously if you a have a peformance engine, you must follow the advice of the builder.
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