Combustion is the well known exothermic oxidation process.
Mineral oil on or off rags, does oxidize, but very slowly, specially
the oils with ani-oxidant additives.
But there *is* an oil that will easily spontaneously combust on shop
rags - linseed oil, specially raw linseed oil.

This is sometimes sprayed inside welded tube fuselages on homebuilds:
it is a clinging cover film to stop rust *and* it mops up oxygen too.

Brian Whatcott Altus

On Sat, 7 Feb 2004 09:45:14 -0600, "Karin Conover-Lewis"
wrote:

This raises a question that I've had for a long time -- re "oily rags." I've
always been taught they are a fire hazard, since they can spontaneously
combust. Yet one cannot help but have oily rags on a boat when doing any
kind of work around the engine, keeping tools from rusting, etc. So what's
the best way to SAFELY store them? The lubberly method of keeping them in
buckets of water won't work for obvious reasons.