the best way to get that oily film out yourself is to repeatedly rinse the cooler with a 50.50 mix of kero and metho. Pour the mixture in one end with the other end sealed and wash it around to and fro. Then pour it out and youll see how dirty the mixture is when it comes out. Repeat another 5 times.
Then, wash out the residue with clean water and leave to dry in the sun for quite a while to ensure the core is dry inside.
This is Sydney kids recommended mixture and it works a treat!