93+ only in the EVO, but in my DD's its cheapie, and a fill up w/ premium every 8-10 fill-ups. The way it was explained to me was boosted vehicles need high octane to burn evenly and thoroughly to prevent possible detonation. That it helped n/a engines with high compression, or high revving engines. Or for non performance cars, just to clean em out every now and than. This is just what I was told so don't quote me.