Kirkland oil looks like good stuff...
Today.
Who knows who they spec & bid to tomorrow? I don't trust their Interstate Batteries, either.
Read an article on them, Interstate is just a holding company for any battery maker under the sun - literally. China, USA, Mexico, you name it. AND - different Costco Stores get different Interstate batteries from different plants around the world.
Might be an MBA case study in how to globally source lowest cost product, but not for me for a critical vehicle performance part. Laundry detergent and diapers might be fine.
Gasoline is different - all retail outlets pull base gas from the regional tank farm - not globally. So buying gas @ Costco is fine, they advertise Top Tier additive levels.
But you really gotta watch Costco on other things. They aren't the holy grail of all product lines. And yes, I'm a Costco Member.