Costco gas is not “bad for BMWs.” That claim usually comes from mixing up cheap prices with low quality. In the U.S., Costco gasoline is TOP TIER detergent fuel — the same program BMW and other makers have pointed owners toward for deposit control.
Follow the octane requirement
Many BMW gasoline engines specify premium. Buy Costco premium when the pump octane meets the manual (often 91 AKI, sometimes 93). Using 87 regular in a premium-required BMW can cause knock and a check-engine light; that is an octane error, not a Costco defect. See 91 vs 93 and check-engine lights.
If your BMW allows regular, Costco 87 TOP TIER is a normal choice and is usually cheaper than branded regular.
Same caveats as other cars
Typical blend is E10. Additives are detergents, not Techron. Membership is required at the pump. Compare nearby warehouses on U.S. Costco gas prices or a hub like Houston.