Tara, having syntocinon is supposed to HELP you deliver the whole placenta intact. It is definitely not a risk factor for retained products. The opposite.

Given your history, I think that they may want you to have syntocinon no matter what. There are some other things you may want to discuss with the consultant. They can check the uterus after delivery to make sure the entire placenta is out. This can be done in Delivery Suite with an epidural, or you can go to theatre and have it under a general. I know it sounds yucky, but then you have no surprises later. It doesn't have to be done in a rush after the baby comes, unless there's bleeding. You can give birth, BF while letting the cord pulsate, and then have the cavity checked.

Definitely ask Dr Bisits or whoever you see, about management of the third stage.