Amazing post @Esox! If I may add a little something:
That looks like the kind of light staining you get on cutlery or cookware, too. Scrub all day and it'll still be there.
OTOH, some water and vinegar and off it goes.
I would say the less one can get away with, the best; dip briefly in little vinegar and some water, and increase dipping time if it's still there. It usually goes right away IME.
I'd personally favor vinegar since it's cheap and easy to source, and probably everybody has some in their kitchen, anyway.
To clean that cap I used Bar Keepers Friend. It is mildly abrasive but you dont use it to scrub with. The powdered BKF when mixed with water creates Oxalic acid. That acid is what does the cleaning. Below, after 5 seconds of rubbing the cap with my wet thumb and a bit of BKF.
That looks like the kind of light staining you get on cutlery or cookware, too. Scrub all day and it'll still be there.
OTOH, some water and vinegar and off it goes.
I would say the less one can get away with, the best; dip briefly in little vinegar and some water, and increase dipping time if it's still there. It usually goes right away IME.
I'd personally favor vinegar since it's cheap and easy to source, and probably everybody has some in their kitchen, anyway.