Costco does not sell a single premium octane rating in every warehouse. In much of the United States the premium grade at Costco is 91 AKI. In some eastern and midwestern markets you will see 93 AKI. The yellow octane sticker on the dispenser is the authority, not a forum post from another state.
Costco’s own gasoline FAQ notes that octane can vary by location. Canada and the United Kingdom use different octane labeling (RON), so a “premium” number there is not comparable one-for-one with U.S. AKI.
Why the number changes
Refiners and terminals supply different premium blends by region. High-altitude states sometimes post lower AKI for the same knock resistance. Costco prices and grades follow what that warehouse can buy, not a national SKU called “91 everywhere.”
What your car actually needs
Use the octane in the owner’s manual. If the manufacturer requires 91 or 93, buy that grade (or higher). If it only recommends premium, regular TOP TIER 87 is usually acceptable and cheaper — the engine computer will adjust timing. Luxury brands are covered in our notes on Audi, Porsche, and BMW.
Compare posted prices on California Costco gas or a metro hub such as Ottawa, then confirm the grade on the pump before you fill.