Just thought I'd pop in, and say that I have used with my first two pregnancies and with the advice of my doctor, I took it in small doses, and drank plenty of water to stay hydrated, it worked with both my pregnancies, but I was already dilated and effaced, ie my body was ready to go into labor...neither me or my babies had any complications, however, it does taste worse than anything I've ever tasted...many women can't stomach it, and will throw up immediately...castor oil would not be my first choice, and I probably wouldn't use it unless it was a last resort and advised by your doctor or midwife. In my opinion, the effect you're looking for can be just as easily obtained by eating foods high in fiber, that will give you the same results, which is looser bowels, which stimulate contractions, but doesn't taste bad, and is prob a lot better in the long run.
On a side note, i see alot of women, saying "baby will come when its ready" I respectfully disagree. Babies do not always come when they're "ready" millions of babies arrive way before they are ready all over the world, so they definitely come before they are ready, and on the other hand, many babies have passed away in the womb because they were in there too long, and the womb was no longer a healthy environment for them to live in, simply because the woman's body never went into labor on its own...this generally is uncommon, but it does happen. I know several women who's baby's never came on their own, so in other words the statement, "baby will come when its ready" is not accurate....I don't mean to be offensive, but its the truth, some babies need help, some don't. Its not so much if the baby is ready or not, its the mother's body/hormones, that control if/when labor will start/stop/continue. Some babies are perfectly ready to come, but the mothers body needs assistance, which in many cases has/will lead to natural or medical induction. With that said, you really should talk to your doctor before you attempt any natural/medical methods of induction to be on the safe side.
As you said you've already done your research, so good job there, I really hope you go into labor hun, and if you have any other questions or anything about my castor oil experience, feel free to PM me...