What has octane Oscar recommended for The IS 300? - Lexus IS Forum
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post #1 of 5 (permalink) Old 04-12-2019, 12:44 PM Thread Starter
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What has octane Oscar recommended for The IS 300?

89? 91?
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post #2 of 5 (permalink) Old 04-12-2019, 01:09 PM
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Lexus recommends using whatever premium is in your area. You will get the best performance from that. Lower octane will cause the ECU to start pulling some timing if it senses any knock.

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post #3 of 5 (permalink) Old 04-12-2019, 03:28 PM
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I use 89 oct and have for 10 years in my DD IS300. That said, most consumers use 87 or 91. Which means the fuel sitting in the 89 oct tank does not get refreshed as often as the others do and that means the fuel may be less than 89. I stop at a truck stop that gets a lot of traffic where I live. When I go to a smaller local gas station, within 20 miles I get a CAT code. I suspect my fuel trims need to dump fuel and that heats up my CATs and trips a code. New gas will resolve the code. A little seafoam help too.

I average 23-24.5 mpg on every tank. Mostly hwy driving. 21-22 city and my engine has 262,000 miles on it. AMAZING!

The "premium only" label is there for CAFE standards to maximize fleet mpg. The compression ratio is at 10:1 and knock sensors will keep the enigine safe. I wonder if the new is350 with 11:1 compression can handle 89 octane safely?

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post #4 of 5 (permalink) Old 04-12-2019, 05:25 PM
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i believe my fuel door says 'premium fuel only' the highest we have here is 91. buy whatever is 'premium' in your area

before my base 94 toyota pickup got stolen all i put was 91 in it. there was a thread on NorCalDrift (which i believe is no longer around), where a guy logged his fuel MPGs between 87/89/91. his ae86 actually was cheapest with 91. his reasoning was that the car doesn't use as much 91, combine that with feather footing and he was able to save a little money between fill ups that add to a substantial amount in the long run. it went on to say that not all cars are like this and you will have to experiment with your own vehicle.

with that being said (and off topic) your driving style is the biggest factor in fuel consumption, IMO.

Lucy; 04 manual sportdesign
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post #5 of 5 (permalink) Old 04-12-2019, 06:04 PM
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Just run 91.
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