What I would do to clean the inside of the used oil cooler. Pour some gas in it and shake it around a couple of times and drain. Repeat that a couple of times until you think it's clean. Now if you are going to use it for the power steering cooler or tranny cooler. Pour some tranny fluid into it, shake and drain a couple of times. For the diff cooler, use the gear lube you was planning on using.
