Blue smoke, but not on first start
 

Hi guys,
I have Civic 1996, and I'm curious why only sometimes blue smoke appears on start up.
Usually not at the first start but after the next hour again when it starts.
Before two years (8000 miles) i was rebuilt the engine ( new bearings, rings, gaskets, new four valves because was burnt).
When i press the pedal to 7000 rpm there was no smoke.
What's are yours opinions?
Oil comsumption is around 500 ml for 3000 miles.