Castor oil is the classic unpleasant remedy of childhood, so tradition reads it as something hard to swallow that is supposed to be good for you. It often reflects a duty or lesson accepted reluctantly.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.