Sintered parts made through powder metallurgy are never fully solid, they retain a network of tiny connected pores left over from incomplete fusion of the powder particles during compacting and sintering. If those open pores are soaked and filled with oil instead of being left empty or sealed with metal, the component now carries its own lubricant inside its body, and friction generated during use draws that oil out to the rubbing surface automatically. This deliberate filling of pores with a lubricating fluid is called impregnation, and it is exactly how self-lubricating bushes and bearings are manufactured without needing any external oiling system.