I always had best luck with Castrol Super Clean in the purple bottle and water.
Pour half the bottle into the intercooler, swish it around really good. Let sit for 20-30 minutes to help breakup caked up oil residue. Then, turn on the hose and let it run in one neck until the junk starts pouring out the other end. EPA peeps please turn your head and look away.
Pour the rest of the Super clean into the intercooler on the other neck. Swish it around for awhile. Let it sit for another 20-30 minutes. Fill that neck with water until it starts pouring out the other neck. Now, with the intercooler completely empty, I fill it with water, repeat the swish procedure, dump, fill again, dump, fill again and dump. I keep doing this until the water is clear. It might take awhile before the water really gets clear, but this is why it's important to let the super clean work it's magic. Once you are satisfied with the cleanliness, turn the intercooler upside down so that the necks can drain out and tilt the intercooler at an angle. Just let gravity be your friend to help drain it out. I always let mine sit for about 4-6 hours. After which time I shake the intercooler as best I can. No air compressor back in the day. If you have one, you can blow it out pretty good using a towel and sealing the nozzle with the towel and your hands.
That's it, that's how I've been doing it for years. Works everytime.
HTH
Patrick