Oil Residue in Tank

We have two Briggs toilets that are about 20 years old. About a year ago, they started having an oil residue in the water tank and created a black ring in the toilet bowl. We have replaced all the pipes going into water tank, and all the internal parts of the tank, but still have this oil residue. We have let water from a nearby sink stand, but have found no residue in it. Does anyone have an idea where this could be coming from and any solutions?