In case of an oil residue at a suspected leak, what action is recommended?

When dealing with oil residue at a suspected leak, using a bubble solution is the recommended action. A bubble solution helps identify the exact location of a leak by creating bubbles that form in the presence of escaping gas or liquid. This method is both safe and effective, as it allows for visual confirmation of a leak without causing any additional risk, avoiding the potential hazardous implications of other methods.

The other choices do not provide the same level of safety or accuracy in leak detection. Ignoring the residue can lead to dangerous situations, as undetected leaks may pose risks to health, safety, and the environment. Using a water spray may dilute the oil but doesn't precisely indicate the leak's source, and closing the fitting immediately could lead to complications, especially if it is urgent to assess the situation before taking further steps. The use of a bubble solution remains the most reliable method for pinpointing leaks in this scenario.