We (S&S/FHP) do run vent holes in the upper intake tins, we look at it this way, the Intake port will create a vacuum thru the guide clearance that would counter act the vacuum for oil evacuation, therfore a vent is needed. The exhaust would create a pressure thru the guide clearance aiding in oil evacuation so no vent is needed. One thing i've wondered is if we vented the exhaust side, is it possible that the vent would help stabilize the air in the exhaust tin so that if there were too much pressure from the exhaust (valve guide wear) it wouldn't put pressure on the sealing joints and prevent some leaking or weeping. We (S&S/Flathead Power) are working on a better sealing solution for the knuckles (using high temp oring or seal similar to what fhsmith1 describes), that may include valve giude seals to take the port pressures out of the equasion all together. This soulition would retro fit on any S&S, FHP or Stock heads with no modifications to the heads themselves. I hope to be testing by mid January.
S&S Cycle, Vintage Lead (formerly Flathead Power)