a quick little breakdown... in a negligence situation... such as this...
in court the appellant (insurance company) would have to prove:
1) that there was a duty of care (in this case, duty to safely operate motor vehicle on roadway)
2) there was a breach of duty (the crash)
3) there was a precaution (slowing down in the turn)
4) possible that precaution (slowing down) would have prevented breach (crash) ***note that they dont ave to prove it would have prevented it, just that it was reasonably possible that it may have prevented it.
5) they are done, you are liable.
if there is something that could detract from your liability, you must bring this up. either the contributory negligence of someone else(negligence of cityfor not keeping roadway clean and safe... but not sure that would hold any weight), or somekind of intervening act (might argue that the oil was an intervening force... but it is non-moving. you hit it. you failed to avoid it.).
this isnt a criminal matter, there is no proving beyond reasonable doubt. they just have to prove that it happened, and that there was available precautions you could have taken to avoid the incident.
so if you could sue the insurance company... then i dont think you would stand a very good chance at winning.
They have recorded an at fault accident where an "at fault" accident did occur. single vehicle collisions are always at fault (unless some sort of intervening act or contributory negligence-someone else does something that causes/contributes to the accident- can be proved).
however, you might stand a better chance bringing an action against whoever informed the insurance company (cc'd them in the bill). but you would first have to look into if the police/fire departments have the right to disclose that info to insurance companies, i'm not familiar with the specific law here, but i have a feeling that there's nothing preventing them from doing this. or else I'm pretty sure I would have picked up on it, as it would probably be very profitable.