There is no seal on the top of the valve guide, I would suspect, that even the tightest fitting valve/guide combo would eventually consume any oil if overfilled above the guide. If you look at the old lister manuals, it shows the oil level in the spring cup below the level of the top of the guide. When warm, the engine heat and spring motion helps the oil to wick/mist it's way up the guides and springs to gently lube the valve shaft and springs. You can see this on the springs themselvs. My springs always have oil from top to bottom, but I don't oil the springs specifically. IMO, this is just one more reason to maintain a good hot head temperature:) I use oil for rocker lube instead of grease, and the excess that runs off keeps the cups full to the top of the guide towers. Any extra is consumed by the engine.
I had thought at one time about rigging a drip system for the rockers off the main lube pump, but I could not find an easy way to drain the excessive oil buildup in the valve spring cups that this would cause, back to the sump without drilling some major holes in the head. Perhaps a siphon/vacume tube down to the crankcase would work, but it seems to lube just fine as it is, with only an occasional squirt of oil from me. heck, I like to make rounds on the running engine anyway, so for me it wasn't worth the effort...
