Porsche is picky about octane, not about whether the station is a warehouse club. U.S. Costco gasoline is TOP TIER. That detergent spec is compatible with modern direct-injection engines. The limiter is the number on the yellow sticker.
Match the manual
Most Porsche gasoline models require premium unleaded. If Costco posts 91 and your manual requires 91 AKI (or 95 RON equivalent in some books), that fill is in spec. If your market’s Costco only has 91 and the manual demands 93, use a station that posts 93 or confirm Porsche’s guidance for that engine. Regional octane is explained in Costco premium 91 or 93.
Do not put Costco 87 regular in a Porsche that specifies premium to save a few cents. Knock control will pull timing; that is the opposite of why you bought the car.
Ethanol and additives
Expect E10, not race fuel or E0. Detergents are TOP TIER, not Techron. See additives and ethanol. For price shopping, open U.S. stations or California.