I recently noticed that my trusty 20 oz Contigo coffee cup, which I mentioned
previously had acquired a conspicuous film from my daily coffees with milk. I rinse it out, but hadn't previously cleaned it with soap.
Motivated by health concerns, I washed my cup aggressively with soap and water, scrubbing with a brush, and soaking overnight. This barely had an effect, with removal only effected by aggressive scrubbing with metal. The inside of a contigo cup is hard to access with any force.
So, instead, I let it soak in a 10 ml bleach + 600 ml water mix for about two hours. And now the coffee cup is immaculate. Bleach is amazing!