MichiganDriver, first, as noted the info is in your manual someplace, and very well might be printed right on your gas cap of filler door. There will be language in the manual that gives a certain octane level as either “recommended” and/or “required.” If it says required then you don’t mess with it. If it says recommended - well, then you can sort of do what you want, but you should understand what it means.
The octane ratings get labeled “regular” and “mid-grade” and “premium” (or similar) - but this doesn’t have even the slightest thing to do with gasoline “quality.” All an octane rating represents is the gasoline’s resistance to combustion. The lower the octane, the easier it is for the air/fuel mix in your cylinder to explode. Higher octane gas doesn’t combust as easily. (This is not a difference you’d notice with a match).
For any engine to run at its optimum, and thus over the long haul to be healthiest and most efficient, the air/fuel mix is supposed to blow at an incredibly precise moment in the engine’s cycle. This is supposed to happen from incredibly precise timing of the spark going to the spark plugs.
The problem is that the spark plug is not the only thing that can make the air/fuel mix explode. One thing that can make it explode, even in the absence of spark from the plug, is just compression. What folks are saying is that you engine is probably a turbo and probably with relatively high compression. So if you use lower octane fuel your air/fuel mix might frequently be combusting out of time - i.e. when it’s not supposed to. This is what engine “knock” refers to (although that specific term gets used imprecisely. Another common name for this kind of knock is “ping”).
Knocking is very bad for an engine. These days engines have knock sensors. They “listen” and tell the computer and the computer adjusts to minimize knocking. So if you do run a lower octane gas and it creates knock problems, you might not notice. Many people do though - they can tell that their engine has reduced power and worse fuel economy. In the end the knock sensors will probably prevent engine damage, but your car wont’ run at its best or at optimum efficiency.
So the whole thing can be something of a gray area. Can you step down without damaging anything? Probably. Can you do it without any ill-effects - mmmmm - maybe/maybe not. But if you actually do the math on the price difference (see asemaster’s post) you’ll probably find out the impact on your wallet is less than it seems.