Is an oil company responsible for filling the wrong oil tank?

Responsibility depends on fault. Fault is based on intentional wrongful acts or negligence. If the oil company intentionally filed the wrong tank (e.g. an employee did despite being told which was the right tank) or was unreasonably careless (negligent) in filling the wrong tank (e.g. any reasonable oil company employee should have known which was the right tank), they could be liable or financially responsible.

On the other hand, if the oil company is not at fault--for example, the homeowner marked or indicated the wrong standpipe or connection; the homeowner provided the wrong address; etc.--then the company would not be liable.