Sounds like your running Teflon valve seals.
They are a stiffer seal and don't have good sealing capabilities for the slight side ways motion that a valve has when moving up and down thus seal can create slight gap during side motion and with the higher vacuum conditions at lower rpms (idle and cruising) when throttle plates hardly open allows
oil to be sucked through intake valve and mixed with fuel and burned.
You wouldn't notice any
oil burning out the tail pipes during higher rpm tromps because throttle plates more open allowing high vacuum conditions to take there effect through the carb.
Chances are you wouldn't notice at low rpm operation either because
oil amount leaking is small but continuous and mixing with fuel and keeping valves clean looking.
Have seen this problem many times and it gets compounded even more when running high volume oil pump because more oil being pumped to the top of head and drain holes in head usually haven't been increased to allow this extra oil to be drained back down fast enough thus creating a higher pooling level in the heads which can come up high enough to completely immerse the valve seals in oil.
Solution to this problem is to change the valve seals to Viton valve seals.
These seals are a special rubber that seal and move with the valve sideways motion.
They were developed especially for this problem.