well, I've used the 87, and there has been no knocking under load or otherwise, at all, so I don't think it's an issue. I think my mileage may have suffered 5%, but I can't tell because most of that was the cold winter. I am averaging now 24.5 on my daily commute. sometimes I put 91, sometimes 87, depending on the spread and what I feel like. Let's put it this way-I've had no adverse affects from 87, so I won't hesitate to use it. Mu pld Cad Eldorado was the same way, it said to use prem, but I had no knocking on that one either, even when I pushed it hard. I've been comfortable though putting 91 in most of the time.