cant figure out pecking noise

Hi,I have 1999 F-150 truck that has a slight pecking noise in it and it only pecks when I change the oil.after about 1000 miles just when the oil starts to get dirty it stops pecking,I have used lucas in it but it still does the same thing.So why does my truck run better with dirty oil and the pecking stops?

Are you using 5W20 motor oil?

yes I use 5/30w in it, thats what it calls for.