Linseed oil--linen--flax--flaxseed--all the same. This is the best stuff there is for preserving steel tubing. The first thing it does after the tube is sealed is combine with the remaining oxygen in the tube. If there is a pinhole or crack, it will seep into it, oxidize into a hard film [as in paint] and seal the opening.

Linseed oil on a rag presents a large surface area to the air [oxygen]--it oxidizes rapidly--this can be called combustion or, simply, FIRE!

To prevent this, exclude the oxygen. Fold or roll up rags tightly, place in plastic bag, seal tightly, and put into a metal container WITHOUT any other combustible material. They should make it to the dump/incinerator, or whatever, without creating any hazard.

Side note: I used to rent out floor sanding machine years ago when there were a lot of wood floors and always told them to DUMP THE BAG AFTER USE! Finally had to make them put up $10 deposit on bag. The old floors had been filled and sealed with linseed oil & diatomaceous earth (Fullers Earth) before being varnished. When sanded off and collected into a bag, the old unoxidized oil was given a very large surface area in addition to a nice, insulating combustible mass of fuel. It usually took 3 to 12 hours to smolder through the mass to the exterior surface of the vacuum bag attached to the sander and burn a hole through it.

I oiled my fuselage completely--without heating oil, feeling of it, hanging vertically or any of that hard effort.--but that's another story.