About.com advises against flushing used cooking oil down the toilet. This method of disposing of cooking oil is likely to clog the plumbing in the bathroom.
A To Z Plumbing & Drain Service notes that cooking oil may be liquid when it is first dumped into a toilet, but it solidifies when it cools down. People who attempt to mitigate this problem by pouring hot water in with the cooking oil only cause it to get a little further down the drain before it solidifies.
Before disposing of cooking oil, wait for it to cool down completely. Pour it into a container with a lid or a screw cap; make sure that the container does not allow the oil to seep through. Close the container tightly, and dispose of it in your regular garbage.