I own a 6.4 and there is nothing in the manual that says you must use a pennzoil 0w-40. I also know of dealers that just put in regular mobile 0-40. The dealer I go to gives me the choice if I want the pennzoil or valvoline. They say neither will void the warranty but they recommend the pennzoil. I choose the pennzoil because I change the oil per the manual and don't feel comfortable going 8k on conventional oil.
100k miles on plugs so there is no point in debating spark plugs. If you put on 75k miles a year and are changing plugs every 18 months then it may factor in. But if you are like the normal person who drives 12k miles a year its over 8 years on the first set of plugs.
Major miss-fires that could damage a cat are what cause the "blinking" an ordinary random miss-fire that is even consistent wont make the light blink. So unless your light is blinking just check it out asap. A worn plug wont cause a "blinking" check engine light. A broken or damaged one may. If you have ignored a check engine light so long that something is damaged in the engine then you deserve it. Ive seen many exhaust valves burnt because folks ignored their check engine light for a rich oxygen sensor. That is way more common than ruined cats because of worn spark plugs.
Way to many people here that give advice or opinions on a situation or product they have never owned or have any experience with. Its just "what I hear" or "what ive read" thats called hearsay.