In the past, Arco was considered Craptane.  I think bad rep was

1) because arco gas was blended with ethanol (which resulted in lower gas mileage

2) the use of it's additives, or lack thereof.

These days, most gasoline has some blend of ethanol in it (burns cleaner) at the cost of fuel efficiency. So #1 is a moot point.

#2, I don't believe there's really factual information about one gas companies additive are "better" than another.

Also, Arco was bought by BP. 

Lastly, Costco buys gas from everyone, who ever is cheaper. Therefore, it could be BP,Shell,Valero,etc.