[kjellsbells]

I think your instinct is right: the dark glass is going to negate any test.

I always assumed that products like olive oil sold in thick green bottles were sensitive to degradation by (UV, sun)light, so if that is true then by definition a casual in store test is not going to work. But perhaps green bottles mean nothing vis a vis light.

I do recall that brown bottles were a thing in brewing to prevent beer from becoming funky by the action of light, but perhaps oils aren't susceptible to that.

[shirleyquirk]

Olive oil absolutely degrades in light. Even in dark bottles. Really high quality olive oil is sold in opaque ceramic containers.

[wil421]

It’s the same as beer. Light degrades the beer or oil. My olive oil research says a lot of cheap oils are already going rancid by the time you buy them. Very good quality bill oils, like Costco’s EVOO, will go rancid before you can use it all.