I figure if you're in a pinch or in some backwater place where they don't have the octane you need, I imagine the car will run on 87, but first chance you get to throw the proper octane in, it should be done.
The way I see it, and read it in some cars' owner's manuals, most modern cars that are meant to run on a higher octane can run on something lower, though it is not
recommended for regular use because over time that
will cause damage. So again, in a pinch, you do what you have to do rather than be marooned in the middle of nowhere, and simply fill your tank with something higher when you can.
While not verbatim, I'm pretty sure our manuals say something along the lines of "use premium fuel, or the highest octane fuel available to you". Around my neck of the woods that is 93, though some places will only have 91, and there's not many, if any, stations left around here that aren't using ethanol additives, so I have to bite the bullet on that one. But I'm using the highest octane available to me, 93. If I end up on fumes in some random place like Yellville, Ark., and they only have up to 89...well, that'll have to do until I can get to a station with something proper.
Take a Jeep Liberty, for example. The manual essentially says the car can be run regularly on 87 without any issues, but if you're going to be doing any towing, that will require optimum performance from the engine, so they recommend using 89 for that. By the same token, you could also fill up on 89 regularly and be just as well.
If someone's looking to cut their fuel costs and seeing if they can live on commuting or joyriding on 87, well they picked the wrong car for that...if it could've been run on 87 regularly they would've built it around being able to do that. A Corolla the Boxster is not, so you have to give a premium car premium fuel.
No offense meant to anyone who happens to be from or near/around Yellville, just picked a random place. 
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