Unkl, At idle and low speeds the pressure (vacuum higher) is lower on the manifold side of the valve guide so the tendency for leakage is greater.When pressure difference is greater across an oriface(valve guide/valve clearance)the flow is increased.
But don't panic yet valve guide sealing can be improved without removing the head and you havn't given it a very long break in chance yet.
Make sure the operating temp is getting up to min 180 deg F,and work it hard as you can, high gear acceleration uphill, very little idling,etc.
Although the lower ring is the major oil control ring, the 2nd and top rings play a part in oil control and they are expanded by the higher gas pressures in the comb chamber when higher loading as in open throttle operation.
don't panic yet.
Maybe I'll have a war story later.
