I think the quality of Costco et al gasoline is going to vary geographically buy supplier. Second tier stations are hit and miss. Sometimes may be perfect but next time not so much. In a sock vehicle unless it is really bad nobody would likely notice. However if you are tuned, especially FI, at a certain octane rating and it isn't delivered it can play knock knock ginger with you.
Have that problem even in the summe here running 94 octane from a premium station. It's good for the most part but has something like 10% rated ethanol but in reality the percentage varies. Apparently there is less oversight on fuels around here than have an ethanol content or something to that effect how it was explained to me from industry folk.
In any case glad you got it sorted and wasn't something mechanical needing attention!
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