Well ... baby oil is a much better choice than vegetable oil ... that's for sure. I'd still suggest mineral oil though, as it is the least ... icky in terms of putting it inside a human body.

That said, I did a little google search and came up with this.

"The Oil Retention enema is an injection of about 4-8 fl. Oz. of mineral oil (liquid paraffin, Paraffinum Liquidum, baby oil), vegetable oil or olive oil. It is held in the colon for 1 hour or more before a soapy water enema is given. The soapy water enema can be very strong because the oil will protect the colon from irritation. The oil coats the lining of the colon, and also softens the bowel contents, making for a spectacularly powerful evacuation. When the oil is in the colon, do not be fooled by a sub saying that it feels like it has the urge to expel wind. If it do so, it will expel a blast of dirty oil with it, and it will be very messy. If your sub must absolutely must pass wind, get it into the knee-chest position for one minute, and then hold a piece of toilet paper over the anus, to prevent the release of the oil. Do not let the sub expel the oil before taking the soapy water enema. This last part is optional especially if you like nappy play. You will need to give at least three soapy enemas to remove all trace of the oil, or dirty oil will leak out of the sub anus for about 12 hours. I’m sure you do not need any more of a hint than fit a nappy and save some soap? Your sub will not be able to stop the leaking."