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When topping off the gasoline in a vehicle that is rarely driven (100 miles/month), is there any advantage in topping it off with a higher octane gasoline?

I've heard stories that the octane of gasoline gets lower the longer it sits. I have no idea if those stories are true. If they are true, it seems reasonable to spend a little more and top off the tank with a higher octane fuel.

But are those stories true? Is it a good idea, in this situation, to top off with a fuel with a higher octane rating than is recommended, with the goal being to restore the fuel to the recommended octane level?

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In the case of topping off, it would make absolutely no difference other than costing you more at the pump. Octane is a rating which would indicate how hard it is for the fuel to burn. The higher the octane rating, the harder to burn. If anything, leaving gasoline for longer periods of time is going to make it harder to burn, thus effectively raising the octane level of the fuel.

Topping off the gas tank when leaving the vehicle sit for a while is a very good idea. This helps limit the amount of air which is in the tank and therefor the amount of water which the gas can absorb. Most fuels in the States are blended with Ethanol, which absorbs water. The more air in the tank, the more water it can absorb (assuming there is moisture in the air). Using a fuel stabilizer is also a good idea if you plan on leaving the vehicle for more than a month at a time.

  • I have been researching whether gasoline octane increases or decreases over time. I have not found any well-sourced information, but I keep on finding posts like the one from sammyg2 (who says he is an engineer for Shell), where he states "Octane will be reduced over time depending on temperature and exposure to atmosphere." (see forums.pelicanparts.com/porsche-911-technical-forum/… ). I'm trying to get educated about this stuff, but there is so much conflicting information. Let me know if you can find good sources of info. – RockPaperLizard Sep 16 '15 at 18:40
  • @RockPaperLizard - Yes, gas (petrol), just like oil, has a lot of conflicting information. As I alluded to, I don't know what it does either for sure. It's why I don't like the forums anymore ... talk is cheap. I can see where the detergent package would be degraded over time, but am not seeing right now why octane would be degraded ... I'm sure it could, but not personally seeing any respectable information which would make me believe one way or the other for sure. – Pᴀᴜʟsᴛᴇʀ2 Sep 16 '15 at 20:32
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    It's amazing that with all this technology and information at our disposal, it can be so hard to find accurate information. Especially in this case, where we are talking about a substance (gasoline) used by hundreds of millions (perhaps billions) of people. Perhaps I will ask the question over at the Chemistry SE in the future. – RockPaperLizard Sep 16 '15 at 21:28
  • Another link about fuel losing octane over time: consumerreports.org/fuel-economy/… – RockPaperLizard Dec 30 '18 at 3:32
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The only advantage I can think of is with a vehicle that has carbs. Often (but not always), the higher octane fuels have less Ethanol. Ethanol left sitting on carbs leads to fouling. So, higher octane, which may have less Ethanol, gives you a better chance of not having to deal with fouled carbs down the road.

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