Pouring frying oil down the drain is not advisable because it can cause clogs and blockages in the pipes. The oil will coagulate and adhere to the sides of the pipes, eventually forming a thick, sludgy layer that can prevent water from draining properly. Over time, this buildup of oil can restrict the flow of water and cause the pipes to burst. Additionally, pouring oil down the drain can contaminate local water sources and disrupt the environment.