The reliability of a valve stem seal (VSS) is directly related to its ability to deliver the correct rate of lubrication. Too little lubrication may result in valve stem wear; too much may result in ‘burnt’ valves and seats. The correct operational envelope will therefore depend upon an engineer's ability to predict how parameters such as seal geometry and material properties change over the life of the seal. This paper discusses the use of accumulative leakage measurements to study the life performance of a VSS as changes are made to ‘key’ design sealing features. A standard design VSS is used for ‘normal’ leakage bench marking. The results are to be used in the development of predictive leakage models that use both close scrutiny leakage testing and continued lubrication theory development.