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The posted statement about octane refers to 3.6 engine the question was about the 3.5 engine, I was told that using the 89 octane would help keep the catalytic converter clean which I have a code of p420 car has 3.5 engine with 75k original miles 2006 model of course it uses oil in between changes.
 

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Hi guys,

When I bought my car the salesman told me I should use “supreme” (91octane) and I never stopped to think about it.

I went through the manual and although I'm sure it's in there somewhere I couldn’t find what the recommended fuel type is. I’m going to gas up today and I’m wondering if I should continue giving it 91.

Once again thanks in advance ;)
The car is tunes for "regular" gas which is not 91. You will get no benefit from putting premium fuel in the car.
 

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Interesting observation on fuel grade selection.

While many vehicles indicate that only regular / 87 is recommended & quite frankly they will operate just fine under normal driving conditions.

However, there are some benefits to using premium gas IMHO.

Fact, from my personal experiences - a close friend has an Acura 1.7EL - great car & great for driving in Downtown Toronto.

She started out using regular & things were fine, I suggested to use Shell 91 premium & she did & by the second tank of premium gone through her fuel economy improved to the tune of an average of 70Km's more driving distance to one tank over using regular gas.

It also improved her power, responsiveness & efficiency - regular gas is just that - regular.

Mid grade gas is mixed at the pumps (usually a pump blend of 70% regular & 30% premium) & IMHO is a total waste of $$$$$$.

For most vehicles, filling up with premium will cost on average between $5.00 & $10.00 more to fill, but again I believe that this minor expense increase is worth it.

Higher octane fuel burns hotter internally, prompts a better spark which equals better burn / combustion of fuel & it also promotes a cleaner operating engine as carbon deposits do build up faster just using regular gas.

I have had the fuel grade debate more times in my driving career than I have fingers & toes & all opinions are welcomed, understood and more than accepted.

I just know from experience that I have, and always will use premium grade fuel no matter what car I have had in the past, have now, and in the future because it actually really does make a difference.

Just my $0.02............................
This is a really old post, but looks like some are still following. I had the 3.5L in my 300M but have a 300s now. The 3.5 L was rated for 89 octane but Chrysler has a video and other information indicating that it runs fine on 87 Octane. even though 89 is what it recommended. I noticed that the car missed at constant speed on 87 and ran better on the highway on 89. Probably because that is what it is tuned for.
 
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