Which gas should be avoided when dealing with oxygen regulators due to its risk of combustibility?

The gas that should be avoided when dealing with oxygen regulators due to its risk of combustibility is oil. When oxygen is present, materials like oil can become highly flammable and even explosive under certain conditions. This is because oxygen supports combustion; it intensifies the burning process, so when oil comes into contact with oxygen at high pressures, the risk of fire or explosion significantly increases.

In welding and cutting operations, the use of oil in the vicinity of oxygen equipment can lead to accidents. Therefore, it's critical to keep oxygen equipment free from oil and grease to ensure safety.

While other substances mentioned might have their own risks, they do not pose the same level of combustibility risk as oil in the context of oxygen use in welding. For instance, water is generally safe and does not combust, alcohol and acetone pose their own hazards, but they are not specifically targeted in the same manner as oil when it comes to precautions with oxygen environments.