If my truck is sitting at idle or if I come down a hill and give it throttle I get a puff of blue smoke.I usually add about 1 litre of oil (1 quart I guess ) every 5000miles.Any ideas?The truck doesnt have too many miles on it 250000kms.If it were valve seals how hard to change?

 

Symptoms sound like guides and/or seals.
You can look and see if any of the seals have ridden up the valve stem off the seat by pulling the tappet covers and looking with a flashlight but its been my experience the guides will need replacing if the seals are shot.