Mixing different octane.. bad?

There’s always extreme cases…but those are rare and definitely not the norm.

It’s the norm for the J series Honda engines (OP has a J35, I have a J32). My knock sensor kicks in in hot weather if I’m only running 91. The computer gets a lot happier when I can find 93. And lest that be viewed as mere anecdotal evidence, the Acurazine forums are replete with others noting the same experience.

It should also be noted that in general, at least where I live, 93 isn’t any more expensive than 91. They’re both listed as “premium,” and have the corresponding 40-50 cent upcharge. So it’s not like it’s costing more money to fill with 93 vs 91, and in fact as OP fills at Costco it’s notable that oftentimes their premium prices are lower than the regular price for the station down the road.

I used to have a car that needed a little more than 87 to not knock under heavy loads, instead of filling half and half, I filled up when half empty and alternated regular and premium for my fill ups. It worked quite well for me.

I doubt that using 87 in an engine that’s meant to use a higher octane would lower your fuel economy by much.
If you remember a thread from a few weeks ago. We discussed how different valve timing affects the dynamic compression ratio of an engine and why some engines can burn 87 even though they have a static compression ratio of around 10.5 to 1.
Well, closing the throttle and reducing the intake manifold pressure also reduces the dynamic compression ratio to the point that 87 can be run without knocking. We’ve all seen this in action when an engine knocks while climbing a steep grade, but the knock goes away when we lift our foot off the gas a little.
Most people trying to get good gas mileage drive as if there was an egg between their foot and the gas pedal and as a result, the dynamic compression ratio is so low that 87 octane will run just fine even in engines designed for 93 octane.
I once ran out of gas with my Chevy Nova and the only fuel I could find was lantern and camping stove fuel. This stuff has an octane rating of nearly zero, but the engine ran without knocking if I kept my car’s speed below 40 mph. Try to go any faster and the engine sounded like a can of marbles being shaken. Lucky thing I didn’t have to drive far before I found a gas station.