Have you tried changing your diet?

KLM wrote in message . ..
There is a black greasy stain on my toilet bowl I have having trouble
removing. I have tried the usual toilet brush, scouring with
scotchbrite pads, steel wool,using liquid soap, detergent, toilet bowl
cleaner formulas, paint thinner and concentrated sulphuric acid. Most
of the stain is gone but two streaks remain as well as a thin blackish
greasy smear over the bowl that seems to only move around when I use
steel wool on them.