[jqpabc123]

Sorry, I don't buy gas at Costco for the same reason I don't buy gas at the local convenience store --- Costco isn't a refiner/manufacturer.

Every "top tier" refiner has their own special formulation of additives for gasoline. These additives help prevent knocks (premature detonation) and control buildup of deposits inside your engine.

Non-refiners tend to buy gas from any source that will sell it at a decent price and mix it all together. The resulting additive mixture is a question mark --- effectively unknown and untested.

This is the reason that auto manufacturers tend to only recommend "top tier" gasoline. Using cheap, "Heinz 57" gas in your expensive ride doesn't make much sense to me.

[dlcarrier]

Fun fact: Costco is TOP TIER approved (https://www.toptiergas.com/gasoline-brands/), just like Marathon, Valero, ExxonMobil, Phillips 66, Chevron, Citgo, Sinckair, and BP.

Also, the rest of those companies refine most of the gasoline in the US, and while they have their own gas stations, they also sell to other gas stations, and their fuel is just as clean, because the EPA requires it to be so, as has has been the case for decades.

Any mixtures need to be specifically tested and approved. The gasoline market it so heavily regulated and consolidated that you can't get better or worse gas by switching suppliers, because there's pretty much no difference, other than regional variations mandated by local regulation.

[jqpabc123]

because the EPA requires it to be so, as has has been the case for decades.

The EPA's concern is the environment --- not your engine.

[dlcarrier]

Fun fact: The better an engine is running, the less it pollutes.