An oil treatment system for an internal combustion engine in one implementation includes an elongated dipstick tube defining a tube axis and a first flow passageway, a housing arranged on the dipstick tube and defining an internal cavity, an oil treatment apparatus disposed in the internal cavity, and a retainer coupled to the housing. The retainer may have a cylindrical body comprising an elongated first flow protuberance insertably received through the oil treatment apparatus to establish fluid communication between the oil treatment apparatus and the first flow passageway of the dipstick tube. The retainer comprises a cavity through which oil may be added directly to the first flow passageway of the dipstick which may be fluidly coupled to an oil sump of the engine. The oil treatment apparatus may be an oil filter. An oil treatment additive may optionally be added to the housing.