It depends on the oil used.
If you use a coconut based oil, or some other oil based on fats, yes, it can easily go rancid, and if you leave it long enough it will harden into a skin-like substance. I have never seen a blade actually damaged by this substance, and it will wash off with mineral turps.
However, if you use something like medicinal parrafin or baby oil as the base, it will not go rancid. If you use light machine oil or a good gun oil it will not go rancid.The light oils such as medicinal parrafin tend to dry out over time, leaving a very light residue that brushes away easily with the next application of oil. The other oils such as gun oil, provide a long term protective film. Neither of these types of oil will cause damage to a blade under any conditions.
It is not really so uncommon for blades to be smoked over menyan, but this ritual is a part of a total belief system. It cannot stand by itself.
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