I have a 2006 CTS 3.6 105k miles. I got a misfire so I decided to change the plugs. Noticed that all of the plugs have excessive oil On them and in the tubes. I replaced the tube seals in the valve cover, valve cover gasket, plugs, coil pack on the misfiring cylinder. It ran great for about 10k now the misfiring has came back and now on multiple cylinders and all the plugs are covered in oil again. The only 2 codes besides misfires is O2 sensor, and catalytic converters. No smoke, no overheating, coolant ok. Any advice?