why is laser printer toner magnetic?
One of the major ingredients in modern printer toner cartridges (for laser printers) is magnetite, Fe3O4. It is used a charge control ingredient, known as a triboelectrification agent. 'Tribo' comes from the Greek meaning 'to rub' (carfeul if you google that one), and essentially refers to the creation of static charge through friction. This is required to have the printer particles stick to the paper. You can read about triboelectrification here. Magnetite, of course, is magnetic, and this is why your toner can produce a ferromagnetic fluid.

Other ingredients in toner cartridges include waxes, polymers and pigments. Many formulations are water or alcohol based and the toner is actually prepared as an emulsion which is filtered and dried. In order to get this material dissolved into kerosene, you probably need to add an emulsifying agent. Oleic acid is commonly used to achieve this, as it is a long chain acid that has both polar (hydrophilic) and non-polar (hydrophobic) properties.