I would try prune juice instead of castor oil!
I'd avoid castor oil. If you really want to self induce labor, use a breast pump. Also, vaginally inserting evening primrose oil will help you to dilate and thin out. Raspberry *leaf* tea is also suppose to help regulate contractions.
Castor oil can make your baby have a bowel movement inside you which can be deadly to you and your baby could swallow it's own bowel could hurt the both of you. I recommend not doing it!!!
You're not due yet. Is there a reason not to wait until the baby comes naturally?
Try to stay away from stimulants and supplements to start labor. Try natural things such as walking, squats, sex etc to help start labor. Never take oils, herbs, supplements without guide from your midwife or Dr first during pregnancy :)
There are mixed reviews on caster oil. Personally, I'd stay away from it. Try sex, walking, bouncing on an exercise ball, etc. But at 37 weeks, you have a couple of weeks and I wouldn't short change baby that time in their warm, safe and comfy place. :)) It's uncomfortable in the end, no doubt. But it is also the last YOU time you'll have as once baby arrives . . . you are ON and a caregiver 24/7. So, read some books, watch some movies, take a leisurely bath (think you can take baths in the end--- I did but something in my mind thinks some say not to), go get a pedicure (good toes for delivery are nice!), lunch with friends, have a date with your partner. Take advantage of the time as babies are wonderful but they DO change the whole world for us. But again--- you can always try sex, walking and bouncing. In the end they check your cervix during examination as well and that always caused me to dilate a bit more. good luck