This is a dangerous issue. Anyone with a shop that has no automated fire suppression and dealing with petrolium soaked rags should spend the 40+ dollars on something like a Just-rite (or another vendor) oily rag can. They are shaped in such a way they circulate air and to disperse heat and also keep from sparks. If there is a fire they stifle oxygen. Small price to pay verses the cost of a shop.
From the world wide web.......
"Spontaneous ignition occurs when a combustible object is heated to its ignition temperature by a chemical reaction involving the oxygen in the air around us. This “oxidation “ process creates heat that, if not dissipated, will build up until ignition occurs. Generally, this can happen when the materials are left in piles and the heat being generated in the pile cannot be released into the air.
The possibility of spontaneous ignition is great if the surrounding air is also warm and dry. The added heat, say from nearby machinery or a non-insulated steam line, can either pre-heat the material, which in turn sets off the reaction, or can hasten ignition by adding even more heat to the combustible.
It is simple to prevent spontaneous ignition, since oxygen is needed for it to occur. Materials subject to spontaneous ignition should be stored in covered metal containers such as a rag safety can or trashcan. Admittedly the container will contain oxygen at first. However, the oxidation process will use up the reaction and the reaction will stop-fire prevented."