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Quite a few discussions about this topic the past few days.
Higher octane fuels is not a "luxury" grade of fuel: it's formulated for high compression engines (turbos, vtec, etc.). Higher octane fuels ignite more slowly to allow for the charged air to fully compress prior to ignition so you derive full benefit from the higher-performing engine you've opted for.
If you use a lower octane fuel, you'll get pre-ignition (knock) which the knock sensor will detect. The ECU will compensate by restricting performance in order to protect your engine from the effects of burning low octane fuels: burnt valves, cracked heads, and toasted piston parts.
The automotive engineers who design these engines are not trying to make an extra buck or two for the oil companies; the octane specifications are there for a real purpose.
If you have a few extra minutes at the pump, and you want to save a few pennies, mix midgrade (89) and super (93) in equal amounts to get 91 octane, the specified octane. Another trick is to fill with super, then at half a tank, refill with midgrade.
BTW, the deposits on your plug don't have anything to do with the octane level of your fuel. At 93k miles, you should expect some deposits on your sparkplugs.
Don't cheat your engine or yourself - don't burn regular in your car.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)
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