i use 87 because i believe that is what the owners manual of my 03 SL AWD says to use. Going up will add more performance, and more gas mileage however, i believe it is all in your head. The price difference between two of the octane levels is so different per gallon compared to the gain in gas mileage. I prefer to save the $2-3 every fill-up and spend it on her oil changes (better synthetic oil). But hey thats just me. And I think the acceleration is all in your head. Yeah, it gives better performance, but so does a K&N air filter. The difference is barely noticable on a dyno, let alone while driving.