Yes, running lower octane is fine for temporary use when the recommended type is not available. It's also true that the knock sensor will signal the ECU to retard timing when necessary. However, it depends on how low of octane you put in the tank, how hard the engine is running and under what condition, intake temp, the fuel pump pressure, the presence of carbon build-up which increases the compression. etc. Because the engine could suffer damage due to the limited degrees of timing the ECU is able to retard and the amount of fuel the ECU is signaling the injectors to put out based on the inputs of other sensors. Also, don't think you can always hear the engine knock without the proper equipment such as a knock monitor, as it can be very subtle masked by other noises in the area.

 

I never thought of that! I could do a dyno with 87 octane, hammer to the floor, ECU freaking out trying to get the timing to about 30 deg ATDC. Hmmm...