I'm pulling this from a post about the J30A1 engine (the one I have):
"Typically a 10.0:1 CR is the crossover point on whether an engine will need higher octane fuel or not. A conservatively tuned engine with a 10.0:1 CR can do fine with 87 octane, but one with more advanced tuning will need premium."
You have a 3.5L Naturally-aspirated engine in your MDX, which doesn't have any supercharger or special tuning [it's conservatively-tuned, in other words]. You're at the borderline for truly needing a higher-octane fuel, so if you find that you're getting better fuel mileage with 100% Regular 87 Octane gas, by all means use it. Don't waste your money on premium fuel. Your engine will run just fine without it. There are no drawbacks to using 100% gas, only advantages.
Also, don't worry about degrading anything in your fuel system. At 10%, you're really not at a high-enough concentration to start damaging or corroding things. If you were to put E-85 in your tank, however, you would damage your injectors and corrode your injector rail, not to mention the horrible performance you would get and other problems too numerous to mention here.
I have a 1997 Acura CL 3.0L that HATES ethanol. Even at a 10% concentration, I get valve clatter on acceleration, less power in the lower RPM band, and decreased fuel economy. If I use 100% 87 Octane fuel with no ethanol, I get 3-4 more MPG overall. (No joke! Corn is bad for your engine).
Mixing ethanol into regular gas was a really dumb move. Politics+industry=stupidity.
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