I am NOT an expert, but I have issues with both responses 2 & 3.
As far as I am aware, metro areas which don't meet fed clean air minimum requirements require all gas to contain ethanol.
The higher octane is there for knock suppression, but, the computers fight knock at lower octane by changing timing which robs performance. The '05 PT Cruiser I had would get slightly better mileage and yield slightly more "snap" with 91 octane but it tested to be not worth the cost premium. To be certain of the answer to your question, I suggest you try a full tank of mid-grade and see how your car behaves and how it affects mileage. You will NOT get better mileage and performance with the lower octane. The manual listing means the 87 grade is entirely sufficient for normal operation.