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You should put 91 or higher. Putting less will cause the car to detune itself so you get less performance and fuel economy out of it. It may also damage the engine in the long term.
It seems like people that buy the IS couldn't care less about performance and only care about the looks and feel of the car. Therefore they would want to save a couple bucks to neuter the car
Sadly I don't see this question nearly as many times on the Infiniti or BMW forums
What's with people buying 45k-50k cars asking about using regular gas on it....???
If you worry so much about paying for gas, don't buy a luxury gasoline car.
To be fair, I think this OP was confused cause the salesman told him he could use regular gas, where as the OP in the other thread knew that premium was required but still was considering using regular gas to save money.
But yes, as the others have said already you should not use anything less than premium for this car.