can run engine in the garage for 40 min,not leak a drop.run the car about 4 blocks when oil hits header and puff of smoke, pull over and let run, oil drops on top of valley pan,wipe off and let run and not a drop. start driving and in 4 blocks same thing happens. recent engine build,440 new valve cover gaskets. leak in valley pan only when driving? any help appreciated!

On a new rebuild there will be more blow-by gasses because the rings haven't fully seated. This is more pronounced when driving the car. If you used an aluminum intake you will need to also use the thin paper intake gaskets along with the valley tray. Factory cast iron intakes don't need this. Make sure you're running a PCV valve and/or breathers on both valve covers. You may need to remove the intake and re-seal with RTV.