Water Tank Oil Residue

I haven't received any suggestions or ideas from my problem posted on 3/23 so I thought I would try another idea. My problem is oil residue in the water tank, creating a black greasy line in the toilet bowl. After I have replaced everything I can think of, I think the problem may be coming from something eroding from the tank.

Would anyone recommend sealing or painting the inside of the tank? If so, with what product?

Do you use a cleaning product in the tank? This cleaning product eat the hell out of rubber components in the tank. Just wipe you hand over the flapper and see if you don't get a gooey black mess on you fingers. Quit using that kind of cleaner and replace any/all rubber parts in the tank.

Are you possibly hooked up to well water~I believe I remember seeing this problem once before with this type of non-potable water ?

I haven't used any cleaner in the tank. I checked the flapper and it is fine - I replaced it along with all the water pipes and fittings coming in, but still have the residue. Also, we are not on a well.