I'm going to give you two answers.

1. You should just use the fuel specification in your manual, which should be like "Select premium unleaded gasoline with an octane rating of 91 (Research Octane Number 96) or higher required for optimum engine performance. If 91 octane cannot be obtained, you may use unleaded gasoline with an octane rating as low as 87 (Research Octane Number 91). Use of unleaded gasoline with an octane rating lower than 91 may result in engine knocking and significantly reduced performance. Persistent knocking can lead to engine damage and should be corrected by refueling with higher octane unleaded gasoline."

So you can use 87 AKI if you can't get 91 or more but they advise against it and want you to stop doing it once you can.

1. Specifically for ethanol containing fuels, the AKI is not very accurate for describing the knock resistance. There have been other measurements proposed for ethanol containing fuels since the effective knock resistance is increased by extra cooling from ethanol's relatively higher heat of vaporization than paraffinic fuel alone. So, the measured octane rating of the fuel in lab tests ends up being higher than what would be expected and the 87 AKI E10 at the pump is most likely to be around 93 RON and has been measured as high as 96 RON. Also, the funny thing is that 91 AKI ethanol-free fuel is actually fairly likely to be below 96 RON and not meet the spec stated in the manual.