c'mon now...u can EASILY rate gas by their additives...7-11 and AM-PM = bad...shell, maybe chevron = good...i used to put only shell in...but now that my baby's w/ me in socal there aren't any shells...so i use chevron...but if u keep switching gasses then u keep introducing ur car to diff additives and thas gotta screw something up...much better to keep it running on the same stuff