Sometimes when they port a head they will cut the guides down some on the port side for better flow, the result is that the guide is shorter and wears faster. But it can make more power for a race engine. The valve stems do get some oil even when everything is perfect, new seals limit the oil as to not drown the stems in oil. Every time I have seen the condition that you are having it has been a damaged or worn guide with some slack then it kills the seal, so check the guides if you pull it apart.