I never suggested using soap in his tank. Dawn has been used to clean oily residue off waterfowl, wildlife & structure with great success. It has been proven safe for these purposes. I suggested trying this as a last resort because he seems to have tried everything else. If it was me, I would try the Dawn to clean the CQ in a 5G pail then rinse well with fresh water, bleach it then rinse, boil it and then see what the results are.
The only concern that I really have with the oily residue issue is, what is the oily residue exactly? Is this a byproduct of the manufacturing process or was this product somehow contaminated by something else? I have purchased 5 50# bags of CQ S-grade and have never seen this issue. Only 2 of those bags were black. They were dusty and left a residue on my hands & pail while rinsing but were never oily.
I hope this clarifies what my intent was in suggesting Dawn as a cleaning agent.