Drain the gas or add much higher octane fuel to prevent knocking and detonation issues.

Manual states that the car must be run on premium fuel.

 

drain it to be safe, don's mess around with octane booster.

 

If your car is bone stock, I don't think you'd need to go so far as to drain all the gas... that's a waste. Like people have said, don't boost, and fill up with race gas. There is a table floating around that shows the values of what proportion of race gas you need to mix with a certain proportion of whichever octane pump gas to balance out to be a certain octane overall. You should be fine, especially if you drive more conservatively. Besides, while the car is made for 91 octane, the manual doesn't say to NEVER use 91 octane, as it's not going to destroy your engine. I believe there was a portion that said only use lower octane gas if absolutely necessary (like if there is no premium), though they also say to keep the revs low. Just think about people in Alaska, whose premium unleaded is only 90 octane.

 

Yeah I didnt drain it. I just drove around and never boosted. just filled with 91 since thats all i can find. better than 87 lol