why does my toilet smell like petroleum products?

The toilet is new. (Installed abut a year ago) and in good condition. The trap does not dry out. It is on the top floor of my house and the other toilets don't have the same problem. I have no reason to think the vent is plugged since I'd doesn't gurgle and the sink drains fine. The shower drains slow but it always has.

If the toilet is unused for 10 hours it gets an oily sheen on the water and smells bad. If it sits unused for a week the water turns black and it smells terrible. The toilet is mounted about 18 feet above the city sewer line (second floor)

The basement toilet has a pump which pumps the waste up about 4 feet before it flows down into the sewer line.

That is all the relevant information I can think of.

Any ideas?

Send me a couple of pics??

Sure. Give me a day or two as I am not there at the moment.

Bad Odours come when you are going to flush in toilet or bathroom If the main ventstack was not built properly and the flushing of the toilet is sucking the trap water out of the closest source for air - your sink.

You can try running a little water in the sink right after anytime the toilet is flushed or maybe there is p-trap installed in sink.Take help or Advise from Expert Plumbers by showing the problem.

Miles, please reread the question. The original poster is experiencing some type of film appearing in the water in their toilet bowl, not odor coming from a dry trap.