First you clean the outside with water or air pressure. Since its a heat exchanger, its should be clear of all bugs, leaves etc. Then you should tilt it and see if any oil drains out. I have seen them 25% full of oil. If not use some hot soapy water- use like Dawn or a dish soap that will cut grease. Let is soak for awhile and rinse and rinse and rinse. It doesnt need to be fully dry to re-install, just get as much out as you can. The hot air passing thru will dry it out. I have done this on a few cars and truck with good results.

good luck

Mike