Baking soda paste, then sunlight

Baking soda and sunlight are the two things that actually shift old grease and oil stains from plastic lunch boxes.

Make a paste with baking soda and a few drops of water, rub it into the stain with your fingers or a soft sponge, and let it sit for ten minutes. Then rinse with warm water. The paste is mildly abrasive, so it lifts the grease without scratching the plastic the way a scrubby pad will. Scratches are the enemy here: they trap oil and make every future stain worse.

If a shadow of the stain is still there after that, put the clean box in direct sunlight for a few hours. UV light breaks down the oil molecules. Sounds like a hippie trick, but it works. Old tomato sauce and oil stains have faded away on my own containers this way.

Dish soap handles fresh grease fine. This is for the stuff that’s been sitting in the back of the fridge for a week. No bleach, no magic sprays, no scrubbing until your arms hurt.

Future you will actually want to use that lunch box again.