The user's manual says that I should use 93 Octane but in California the highest is 91 due to emission standards and 91 octane would be fine temporary.

In the long run do you think it will ruin my engine? or is 91 octane fine?

 

welcome to the lovely state of CA...

even if you were born here, the # of people with cars requiring premium gas (so it's been said) caused the gas companies to lower premium gas from 92 to 91 some years ago...

but back to your question...

it's fine, maybe a little less power and/or torque, but it runs just fine.

 

just make sure you clean the carbon out by driving WOT at least once a day.

like CHESTON said, welcome to CA and it will be fine as long as you dont use anything less than 91

 

If you have the money, invest in the Cobb Access Port. Its a hand held device that reprograms the ECU to run different maps. You could run Stage 1 91 octane map and it would also give you additional power. If you really want to be safe, you could have the ECU custom tuned with the AP by a local tuner...your car would run MUCH better after that.

 

^^^
very good point

 

or just drive by and fill up a local station if there's one near that carry 100 octane
2 gallons will do wonders