Valve Stem are those sealing device engineered in such way to allow small amount of oil to pass in their sealing lip.
Here main objective is to achieve controlled flow leakage to allow the oil remain between the valve stem seal & valve, for lubrication.
As its controlled flow, if too much oil is passed coke get creates on valve & causes reduce in engine power or failure. On another hand if too small amount of oil passed, there will be increase in friction & causes valve failure.
These seals are usually available in there standard sets.
Eg. Set Of 4, 6, 8, 12, 24 Or as reqd.