Well, I worked once as a chemist of sorts and "made" all sorts of hydrocarbons. Those lighter than c4 evaporate nicely. Heavier than that and they become "oily" - for lack of a better word. I think it's the oil that you find helps in the restoration of the brush and it's that oil that would remain behind after the flash off. I think you're wise to rinse that away the next morning. I don't like wet brushes even turp wet ones.