There's a roundabout by Sainsburys, Pepper Hill in Kent that when it rains always seems to have an oilfield of Diesel all over it. I'd go so far as to say black ice has more grip, but loads of fun...
Probably is diesel from over filled/fueled Sainsbury lorries. A poster (who is a truck driver) from another forum I visit said that's it's quite common for lorry drivers to overfill their tanks and therefore spill diesel when going round roundabouts etc.
If you're going to experiment with TC off, don't be a twit like me and do it on the circuit until you bugger the rear end into the gravel. Do it on a wet car park, with nothing to it. Gentle inputs and get ready to go lock to lock quickly. Great fun.