It helps to transfer the heat from valve stem to cooling water. Generally there is a dry-slippery friction and changeable heat distribution between the valve stem and guide. If oiling is not  enough,valve stem gets abraded more tahan guide. If abrasion happens both of them, materials are not suitable. It is very important the choice of  appropriate materials for valve stem and valve guide. When the concentricity of the valve seat diameter to the  valve guide bore is wrong, valve stem's abrasion is unusal and one sided.

    As the heat is transferred from valve stem to cooling water, the clearance between valve stem and guide is important. If the clearance is too ample; valves are heated to high temperatures and vale deformation will beunavoldable. Also oil vamish and oil carbon formation happens at the valve stem. If the valve guide is too narrow, the stem wear can be so instensive that the valve function is disturbed. The clearance between valve stem and guide is designed by engine manufacturer and should be advised value in workshop manuel.