I can't think of an issue that causes oil to leak into the combustion chamber from the heads that will stop once it "breaks in".

Bad seals won't start sealing and if it's excessive guide clearance, I suppose it could coke up enough to stabilize, but I doubt it.

Based on your comments regarding #8 cylinder fouling badly and has the highest compression reading I gotta wonder how carboned up the combustion chamber is.

If you do pull the old seals inspect them carefully for evidence of failure and check the valve to guide clearance, just see how much the valve stem wiggles ought to be enough.