A couple of things come to mind. The pushrod tubes may be loose. See if they turn/rotate. If so the head nuts my be starting to work loose. If not loose, check that the valve cover is on correctly and secure.
Is it on the same side as the oil cooler? Oil might be blowing by the seals while the engine is cold and then as the engine comes up to operating temp the seals warm up and stop the leak.
Allen Caron
VW based 53MGTD - "MoneyPenny"
"If one thing matters, everything matters" - from the book The Shack