I was referring to the previous comments about types of oils.......motor oil does not do this.Actually, it doesn't necessarily only happen with oils. Wet a cargo of coal and as it dries, poof. Many a ship has been lost that way. Other materials which could spontaneously combust;
- Towels and linen, during laundering and drying.
- Haystacks.
- Green waste piles and compost.
- A number of chemical substances, such as cellulose nitrate.