Symptoms point to valve stem seal leakage, or worn valve guides. Mostly intake valve stem seals will allow oil into the intake when vacuum is high, or given time for oil to seep down the guide around the stem.
If you did not replace the valve stem seals, it would be a decent bet that doing so might cut out the blue smoke on startup. You can do it without removing the cylinder head. Feed a length of rope into the spark plug hole, and then rotate the crankshaft to have the piston hold the valves closed. Pop off the rocker arm and valve spring, pry off the old seal and put on the new one. I'd apply a bit of oil to ease the seal over the valve stem tip. Replace the spring & keepers, and the rocker arm. Remove the rope, replace the plug, and see what happens. You will have to insure the crankshaft does not rotate once you have placed the rope and blocked the valves from falling into the cylinder. Clamp the flywheel or the pulley on the PTO.
tom