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Company does have some effect.

Top tier gas vs non-top tier.

Many top-tier vendors have proprietary additives and refinement processes to differentiate themselves from others..

Shell seems to contribute to excessive timing corrections in the late model ST’s whereas it clearly works great in other engines.
 

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They are probably about the same. The increased octane probably comes from the 10% ethanol. I would always go with the E0 whenever possible. There is more available energy in a gallon of 100% gasoline vs a gallon of 90% gasoline and 10% ethanol. The MkIII can adjust the timing etc.. to utilize the extra octane, but I would still go E0 91 for the above stated reason.
 

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I would too if the car seems to run OK. Getting away from any ethanol will likely save you at least one fuel pump change over a long car life and there are other benefits too.

I'd bet as well on the only difference as being the ethanol addition, it should drop your mileage a bit. And, once they say 10% you really have no idea. They get more fuel sales by bumping the % past that and here 10% can mean up to 20% real world. They add the ethanol locally and they often tip the % as the ethanol is cheaper than gasoline so they make more profit bumping the amount up. Them winning both ways? Why wouldn't they do it? Pretty sure nobody ever checks for that % either like the state here (Texas) they don't and why the amount goes off the charts. The only way we know is because the new car guys test for it in warranty work and they report it as high all the time to make car owner pay for warranty repairs.
 

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Which ever is top tier is better, and if both top tier, you'll likely pay considerably more for the non-oxy. How do the prices compare? Here in MN pumps typically have 87 E10, 89 E10/E-15, 91 E0. I never see 93 in this state. I use 91 non-oxy (usually called premium here) in my motorcycle and car with turbo, 87 e10 in my focus. If the gas with ethanol will be sitting more than a month or so I put stabil in it. The ethanol in gas does have some cleaning properties itself, at the cost of possibly reducing mileage a bit (usually not noticeably) and concerns about gelling and attracting moisture leading to corrosion.
 

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Yes, non-oxy is higher, the gov't subsidy on ethanol making it almost always lower in price than straight gas is why they tilt the % locally, it makes more profit to bump the % up. Ethanol if the gov't dropped all price support is still from $6-$7\gal. Why when Trump decided to drop the subsidy the ethanol industry stopped it, they would have been out of business overnight. The business is non-sustainable without the gov't paying for it.
 
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