We have to rinse plastic cups before use because of an oily cloudy substance that is there otherwise. What is that?

They are like the picnic/ party/ beer pong throw away cups people usually get in bulk when needing a lot of cups quick. I’ve looked around online but haven’t found any info for as to why the water is cloudy and oily if we don’t rinse them first. After they are rinsed all is good unless you let them sit for hours, in which case it gets a bit cloudy again. What is up with this and should we just toss the lot of the cups? Seems like a waste but also wondering about poisoning. We have them in our house and use them from time to time when grabbing a drink so I’ve experimented on it. Without rinsing (our water goes through a reverse osmosis system for filtering) the water has an oily film on top and is a bit cloudy. After rinsing the water inside the cup is a bit more clear but over time gets a bit cloudy again.

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