What cause blue smoke on startup in a diesel truck

Hello, That is very typical of oil pooling on the back of the valves due to leaky valve seals. Level of urgency is up to you. If it is light and there are no other issues, I would wait until I felt like fixing it. If it is a heavy plume that takes a few seconds to stop... probably have that fixed now. The sump on your truck probably holds 15 quarts of oil, so that wouldn't concern me so much. The turbocharger, however, would. I'd encourage you to pull off the tube running from the turbo to the intake manifold, and look for oil. A little is ok, a lot is not. Don't forget to wiggle the turbine shaft. You should have nearly no play when there is no oil pressure.