I'm trying to think of a test you can do that doesn't carry a risk of doing damage, or the risk of screwing something up while you try and figure this out. You could change your
oil, look for shiny stuff, maybe even cut the filter open, and if it comes out nice and clean, which I think it will, then just change that side's header gasket. If the
oil comes out with a bunch of shiny stuff in it, then there's a problem that will require professional assistance.
