What should one never use on gas cylinders, regulators, connections and hoses?

The correct answer to this question is Oil. Gas cylinders can hold oxygen under pressure. If mixed with oil, the bottle can explode or start a fire, causing injury to property and humans. Regulators are control valves. These valves reduce the input pressure of a fluid. This reduction goes to the value one wishes the output to have.

Regulators are used with liquids and gases. If used with oil, it can also cause a fire. Gas hoses are used in welding and cutting metal. A twinned or double tube would join oxygen and fuel hoses together. Like the cylinders and regulators, these hoses coming in contact with oxygen can cause severe damage.