I dont know if our situations are related or not but I bought bronze guides from livernoise to put in my heads and the o.d was larger on the bronze guide versus the oem iron guide so i ended up getting told from their tech department to bore out the factory guide hold to accomodate the bigger guide and i had my machinist use livernoise specs, once the heads were put into service i had the same issue you were having, chased the problem forever. thought it was stem seals,changed them, no fix, thought maybe the stem to guide clearances were off, that was fine as well, I decided to do a little more porting in the pockets of my head and went to knock the guides out and to my surprise they came out VERY easily! when I put them back together i used a loctite high temp sleeve retaining compund on the guides when i reinstalled them and I havent had oil in my exhaust ports since. How long that loctite will hold up I dont know but all I can figure is there wasnt enough interference to keep oil from coming down between the head and guide when it was at operating temp and the head was fully expanded. But thats just my