Pouring oil onto a choppy sea reduces the wave action. Apparently it really works and there's a demonstration and an explanation of the physics here: https://www.youtube.com/watch?v=f2H418M3V6M This is the source of an English idiom, "to pour oil on the troubled waters." An example of use is "at Thanksgiving dinner, my father used to get into political arguments with his brother-in-law, but Grandma was always ready to pour oil on the troubled waters." In this case, it means Grandma said calming things and stopped the argument.