Oil in all 4 spark plug wells

Was driving home tonight and a small engine fire started on my exhaust mainfold. I got the fire out and noticed oil on the valve cover. Took all four plugs out and there is oil in all 4 spark plug wells. Thought it may be the spark plug seals but I doubt all 4 went out on the same time. Maybe its the valve cover gasket? Anyone have any ideas....any help would be appreciated.

Thanks!

usually when you change the valve cover gaskets you also change the tube seals...