Yes. Costco gasoline is not “plain” unadditized fuel. In the United States it is licensed TOP TIER™ detergent gasoline, which means the additive treat rate is higher than the EPA’s lowest detergent standard and has passed the program’s deposit tests.
What those additives do
Detergent packages limit intake-valve and fuel-injector deposits. Automakers created TOP TIER because the federal minimum treat was not always enough for modern engines. Costco’s Kirkland Signature gasoline meets that retailer program; Costco does not publish a consumer brand name for the chemistry the way Chevron publishes Techron.
So: additives, yes. Techron, no. Shell’s V-Power package, also no — see Costco vs Shell.
Ethanol is a blend component, not a detergent
Most Costco unleaded is still typical E10. Ethanol is a renewable blendstock, not the TOP TIER detergent. Details: is Costco gas ethanol-free?