When oil, especially petroleum base, come in contact with the rubber in the braking system it causes it to degrade rather quickly. I've seen extremes from just leaking to being swelled up 3 times its size from oil contamitnation. Getting it on the outside is fine, it's getting inside that's the issue, especially since it can be very difficult to remove from inside the lines. I suppose if you flush the line with brake clean, then blow it out, that would be ok. But every manual that I've ver seen on the subject states to only use brake fluid as a lubricant for flaring. Personally, I do mine dry, and have never had a problem with it.
