First of all, don't drive a car with coolant in the oil. It will brake down after some time. (may take time, but wear will increase) I would guess (like porscheboi928s/c944t/n said) it is the seal in the oil cooler that has to be replaced. Check this old topic:
http://www.pelicanparts.com/ultimate/Forum5/HTML/001005.html
/Magnus