Check the octane level of the Costco premium gas. According to the post above from @LoudThunder, your RDX runs best on 91 octane, and that is premium, not mid-grade. Some states have 93 octane instead of 91 for premium, and you should use that. Mid-grade is almost always 89 octane and while your car can run on that, it isn’t optimum. Here’s a list of octane levels for gasoline by state. I’d continue to use Costco 91 octane premium.
People might not care about the performance difference, but they should care about the fuel economy difference. I can’t see paying less per gallon just to turn around and use more gallons.
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Fuel economy difference with regular/premium is usually less that 1 mpg. A fuel economy improvement of 0.5 mpg is important to vehicle manufactures to meet CAFE standards.
Similar Honda vehicles are certified while using regular gasoline, the MDX is built on the same platform as the CR-V but they do not share the same displacement engines.
The Integra (premium) is related to the Civic (regular), both use 1.5-liter turbocharged engines, fuel economy shown below.
I have never owned a vehicle which is designed for premium fuel, nor would I ever buy such a vehicle. However, if for some reason, I did end up with a vehicle which requires, or even recommends premium fuel, I’d pay the additional cost to use the recommended fuel.
That’s funny cause I built one just to run on 93 octane lol, as well as have the Infinity I35 that runs on 91, but we only have 87, 89 and 93…It runs better on the 93 then the 89…
But I made sure my Tacoma runs on 87 before I got it… 
I would love 93 non-ethanol if TopTier.
At my local station saw a boat dealer filling up one of their demonstrator boats. Asked him about gas, the engines were designed for regular ethanol gas but run much better with non-ethanol.
From your owner’s manual.
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Likely that engine damage will not be covered by warranty.
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Engine damage from gasoline with octane rating less than 87, doesn’t seem like a real concern. The OP considered using mid grade gasoline.
Here in NH non-oxygenated gas is at marina’s or near raceways (and they have a sign the vehicle must be towed on a trailer for use pump).
I’ve been using oxygenated (10% ethanol - 25+ years ago it was MTBE) for decades. Never had a problem with any engine (auto or lawnmower/snowblower).
If I owned a vehicle that required premium, then I’d use premium. There are knock sensors to retard timing if knocking is detected. But there’s a possibility (extremely slim) that the knock sensor could fail…then you if there’s engine knock under hard acceleration - good possibility of destroying an engine.
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If/when a knock sensor goes bad the ecm should retard the timing to a pre set limit, to stop engine damage… The ecm should be able to read if the knock sensor is working at start up… But I would not want to run it like that for an extended period of time either…
When my buddy installed a new clutch in his Mazda, he broke the knock sensor (somehow) and I looked at the timing and it had pulled (retarded) a lot of timing out of it…
I love how some states restrict the sale of NE-gas like it’s a drug or something! Here in the land of Dixie (NC) anyone can buy non-ethanol gas and use it in anything! Also this “Oxygenated” thing? Down here it’s just called Non-Ethanol.
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Turbo 2.0 motor likes premium. Friend got rid of his linc mkx and got a newer navigator. Has turbo v6. Also premium.
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Same in TN…
At least we can agree that the op should not use regular.
How is the octane rating increased to prevent early detonation?
Like we never had high power engines in 1968 before the famers discovered the profits of ethanol? Apologies to my farmer relatives who have more money than I do.
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Back then we had lead. Now they can use toluene to increase octane ratings for ethanol-free gas. It has an octane rating of about 120.
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What is the price differential between toluene and ethanol, if you know @texases? I’m interested in whether it’s cost effective, so the cost is per gallon of gasoline or some other convenient standard volume measure.
That I don’t know.