Hanks custodian bring's up a good point. Do you have the original valve covers that are vented at the top rear of the valve covers?
If someone has swapped valve covers to the newer "closed" not vented , you should have ( if you do not already ) a vented oil fill cap on your valve cover. Otherwise it may be possible that you are creating positive crankcase pressure and maybe it's pushing the oil out the seals.
I believe it is the original valve cover. I also have a vented oil filler cap.
