I always use 91 octane or 93 if in the US in my SC. I often use mid grade in my ES330 and premium occasionally. It has 250,000 Km (150,000 Miles) and runs like a top. I have been unable to see much difference in mileage.
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On the ES it says to use premium 91 octane or better. Different regions of the US offer different octane values for regular, mid-grade and premium because you cannot get the same octane value in the Rocky Mountains that you can get on the California coast. If you want the fuel economy and/or performance on higher compression vehicles: such as Lexus, then you should not ever use less than what the posted octane values on the pump meets or exceeds what Lexus says is for your vehicle in whatever region you are fueling up in.
You guys do realize that the 4GR FSE is the same engine that is used in the Mark X model in Japan, which is basically the Japanese version of the Toyota Camry. Now I wonder. What kind of fuel is used on a Toyota Camry??? Hmmm...
Two reasons: In my local area There is available no-alcohol gas 87 octane. I would rather have real gas with no ethanol even if the octane is lower. If I owned an IS 350 with a 11.8 compression ratio maybe but the GX is 10.2 and it runs fine even under load.
We have to consider the role of a 26 year old Vassar grad with a Harvard law degree and her influence on this requirement. If testing revealed even an 1/8 of a mile per gallon increase for premium over regular then Lexus would require it to satisfy this learned EPA employee who would delay licensing for any tiny uptick in the Lexus overall mileage number.
This employee is also the reason I could not get a diesel. She thinks they are "stinky".
My dealer told me to use onlypremium gasoline. Thats exactly what teh Toyota dealer told me to use on the Land Cruiser too( which I dont I use costco gas). But this IS seems like its a high performance car. Should I really only use premium gasoline only ? And what gas do you guys use ?
The manual for my IS-350 clearly states in the Specifications section that "if 91 octane gasoline is not available you may use gasoline with an octane of 87 (regular) or higher"
So the car accelerates faster with high octane fuel but it doesn't knock without it. Likely due to some kind of anti-knocking sensor/feature that adjusts the engine to prevent knocking. Google "premium gasoline scientific american" and "premium gasoline lifehacker" to read more on the difference between "recommended" and "required", etc