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After two months, I gave up and replaced the galvanized with PVC.

I realize the PVC isn't an option for you, but perhaps a different
source for your galvanized or iron pipe would do it. I had never had
problems like this with galvanized before.

It may be wise for the OP to replace the galvanized and everything
beyond it, as the oil will have coated what it touched on it's way out.
Yuck. Given that it could be coated with waste oil, it may not be the
healthiest stuff to ingest.