Have an 86 300E with 254000 miles on it. The plugs are fouled out after 3000 miles. It doesn't smoke in the mornings when cranked. Valve seals replaced at 159000 miles and head reworked at 173000 miles. Compression test shows all cylinders at 195-205 range, with very little change adding oil to each cylinder and rechecking. Would replacing the valve stem seals help or is the problem much deeper?