KLM wrote:
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.

The stain follows the water flush "track" from the toilet bowl. I
think it is from the disintegrating rubber from the flapper valve or
the rubber seal between the tank and the bowl. They're are 20 plus
years old and due for replacement.

Is there a rubber solvent I can use or is there some method I have not
thought of?

Stop using abrasives and acid. Black stain that "moves around" would
seem to be an adhesive or tar of some sort. Crazy place for that. Only
other black stain I would expect to find in t. bowl would be mildew.
Bleach for mildew. If that doesn't get it, empty and dry the bowl and
try a bit of acetone. Mineral spirits should remove tar. Is this a
relatively new toilet? Have you changed the flapper? Use a toilet bowl
cleaner? Do the kids write with crayon in the t. bowl?