One big reason, the exhaust valve is usually submerged in oil. Get the best valve seals and go over the drain backs in the corners of the head, there's a lot of oil leaks in the rocker assembly up there. At speed I wouldn't be afraid to say 50% or more of the oil capacity is up in the heads. Get a piece of welding wire 1/16 , bend and attach so the end sticks down a little in the center of the hole, this helps promote flow since oil can't swirl in. Can't remember who told me that but it was way back before the turn of the century.