The original valve covers are defective and truly a bad design. This is common on the 3.5 v/6 engines from 2002-2006. Once the leaking starts around the spark plug seals it can seep into the engine down through the plugs. The burning oil smell is awful coming into the cockpit. There is a sparkplug seal kit but it does not work on the original valve covers, only on the replacement of new design. All Nissans engineers had to do is have a removable plate where the seals are located so they can be replaced but instead they deteriorate rendering them useless and the owner has to replace the whole valve cover (left & right). I'm not sure if this was intentional or just another engineering blunder but it's costing the owners an unwarranted expense.