The duty of the valves is to regulate the amount of fuel and air mixture allowed in the cylinders for proper combustion. While the valves are made with guides or sleeves to keep combustion gases from escaping through them, the seals on the top of the valves keep oil in the valve cover from being sucked down into the engine. Seals are made of high-strength rubber and fit over the top of the valve stem just inside a small collar. When the valve seals begin to wear or fail they do produce some obvious and unique signs like;http://www.automobileglobe.com/how-to-know-bad-valve-oil-seal/