Where did you get the info about Shell & Chevron are low quality of gasoline? What is ACN91?
Here in California, we only use 91 octane (maximum). Other states use 93 octane. That doesn't mean it's low quality gasoline. No, I don't think these gasoline come with 15% ethanol. We only use up to 10% ethanol.
We once believed that ethanol was harming HPFP, but now it's no longer issue. Our HPFP is doing well in the recent years, so it doesn't really matter if we are using 10% ethanol. Some crazy Bimmer drivers put E85 in their cars (some say that E85 is an equivant to 100+ octane).
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