I had the same question and did my own "experiment." When the car was new, it ran fine on Shell 87. After 5000 miles (ECU changes A/F for some reason, I have the CANScan unit to check), it pings under WOT at high RPM with the same gasoline. So I switched to Shell 89 and was fine. I did the 87 experiment again few weeks later with the same result. When down to half tank, I mixed with V-Power to prevent too much pinging. These days I ran Shell 89 and is fine. But who knows, at 10000 miles, maybe only 91 will work

 

Did you try disconnecting the batt, doing a full reset, then doing the brake test to do a soft reset?