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I just picked up my 01 Is300 and I was wondering if I should use 87 or 91 or does it matter?
 

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Cherisher
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Inside of your fuel door should say something like "Minimum 91" or "Premium Only" or something like that.

If I remember correctly, the reason is because of the engine's somewhat high compression ratio.
 

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I asked a similar question a year ago:

Bottom line is, the owner’s manual (even the fuel door) RECOMMENDS you to use 91 octane. You can run on 87 octane, but you will not get a lot of performance out of the engine. The engine will retard the timing from its knock sensors. You will also get bad MPG.

I’d say fill your car up with 91 octane and don’t cheap out on gas. Use 87 octane if you can’t find 91. Otherwise get a Prius/Corolla/Civic if you’re looking to save on gas or a Ford Crown Victoria if you want a car with RWD that takes 87 octane as recommended.
 
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