Actually, the opposite is true. Higher compression ratio = higher chance of knock, so it would make more sense to use higher octane fuel after the carbon has built up, not before.I
Do NOT use 87 and for quite a si.pke reason: the ex engineers at Kia have determined that 87 isn't recommended.
While the car can "correct" itself for the lowest octane fuel, it'd be foolish to do that with a GDI at the start. Once you log several tens of thousands of miles, then a lower octane MAY be ok as the carbon buildup in the combustion chamber will increase the compression ratio.
A recent AAA report found that American motorists wasted $2.1 billion dollars in the last year buying premium gasoline for engines designed to run on regular gas.
And while Kia has "recommended" 87 octane fuel for max performance purposes, the 3.3L in no way requires it. Otherwise the manual would say 91 octane is required.