An internal combustion engine valve stem seal is actually a precision lubricating device that allows enough lubrication to the valve stem for extended life, but limits the lubrication to provide compliance with exhaust regulations, limits valve deposits, and prevents uneconomical oil consumption. The oil leakage for lubrication can be determined using four seal lip design factors. The long term stability of the oil leakage is provided by selecting polytetrafluoroethylene (PTFE) and fluoroelastomer (FKM) rubber as the seal materials, which are bonded together during vulcanization. The PTFE provides a low friction. low wear surface, and the FKM rubber provides a stable resilient body.Validation of the design factors is accomplished by precise measurement of oil leakage using actual engine components. After long term engine testing in the laboratory or field, the leakage can be re-measured. The leakage can be expected to increase only slightly.