I guess what happens is the pressure on the slack /lifters causes them to leak down.
Running the engine with pressure refills them however it may take abit of time,as you have to expell the air which enters them. It may take afew seconds or up to a half an hour. I'll just add this, on one make of v6 I know it wasn't unusual to have to run it
them for 20 minutes or more. Seems idle works better that high speed and I can't tell
you why. Turning by hand would not help as you don't have oil pressure