Old Valve Cover Gasket
A valve cover gasket is between the valve cover and the engine's top. The valve cover's job is to keep oil from escaping from this union. Over time the valve cover gasket wears out, becoming brittle and shrinks due to the heat. When this happens, oil begins to leak.
The spark plug tube seals help protect the spark plugs from becoming contaminated with oil. The spark tube sales are typically replaced simultaneously as the valve cover gaskets.
On some vehicles, particularly some Nissans, the spark plugs tube seals only come with new valve covers, which can be expensive.
If you're smelling burning oil, notice consistently low oil levels, feel a misfire or feel as if the engine is running rough, ask a certified technician to inspect the valve covers for any signs of leaking.
There is no set recommended time to replace the valve cover gaskets. Periodic inspections are critical for overall vehicle health.