This may seem like such a simple, non-technical item, but I never noticed until after the engine was cleaned to better pinpoint oil leaks.
The oil filler cap ! Oil seeping out around the cap and cruding up the engine again.
The gasket and the tensioner had weakened.
(Afterall, it's the original metal cap, now 18 years old).
The new cap, made of plastic, went on much tighter. No more leaks, at least around the valve cover area.
Now I'm wondering if the leaky cap was causing other small problems (from air leak).