My last truck had a 400 bb that when hot had about 0 psi on the gauge on the dash. never had lifter tap or rod noise.it was a work/plow truck and it worked hard. ran that way for YEARs. after the truck rotted to death around the engine I scraped the truck and kept the engine. I pulled the pan to check the crank and found a bad main on the crank. I did a set of new bearings and put it in a 67 satilite and it is still running good just low psi at hot idle.just keep running it. If its not knocking just run it . When it starts knocking then worry.