The gas that must not be exposed to oil or grease is;
Oxygen must never be exposed to oil or grease because of its high combustibility.
When oil or grease which can get ignited easily, comes into contact with the metal cylinder that holds the oxygen, they can cause melting and burning.
This can set off quickly in an explosion and spread to other equipment thus causing a wildfire.
Even some compounds that are not normally combustible can be ignited when they come into environments that have an abundance of oxygen.
Summarily, the gas that must not be exposed to oil or grease is Oxygen.
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