[Dave at Pelican Parts]

High-pressure solvent would probably be the best way. You'll need an adaptor plate--you may be able to use an oil-cooler block off plate for that. High-pressure air can find the leaks in a safer way than spraying solvent out any leaks, but it won't clean very well. Solvent will. Push it both "forward", the way the oil normally goes through, and "backward", the other way.

A soak in solvent and some brushes and pipe cleaners can help clean the outside of the cooler.

--DD