An oil-conditioning filter, for use with an internal combustion engine, includes a mechanical filter element and a central additive cartridge, for gradually dispensing an oil additive over time. The additive cartridge has a beneficial oil-conditioning agent contained therein. The additive may be provided as a solid block within a storage chamber of the cartridge, or may be a plurality of pellets. Central placement of the cartridge allows for fluid filtration through the mechanical filter element before the additive is added thereto, reducing the likelihood that the additive will be filtered out prematurely upon its initial release. An additive cartridge for installation in an oil filter is also described, including a baffle portion and a storage chamber. Alternative exemplary embodiments of the additive cartridge may include a cylindrical shell, a basket which is operatively attached to an oil filter center tube, or a porous center tube.