I went to Virginia Beach 3 years ago and visited the Edgar Cayce Institute...sure wish I had had more time, would have loved a treatment there. At any rate, I purchased castor oil from there and it was used a few times and then ended up on a shelf forgotten. Well, I found this site and have checked out many of the forums and saw something about using castor oil as a flush. I pulled out my castor oil and used it for just that. I had no issues with it, it worked seemingly as well as the person who had posted the thread.
Also, looking at the bottle, there is no expiration date, either.
Consequently, I would have to answer no, castor oil does not spoil.