The poster above is right, those plugs are known to cause misfires in MLs. Likewise, changing them extra often is not preventitive of any major damage type problems. Changing the oil more often can be, but with Mobil 1, every 5k miles is also big time overkill. Also, I do not know where you buy stuff, but an oil and filter change with that stuff costs more then $50. Every 7500 miles for an oil change is fine.

You should go back to OE make/model plugs and leave them in there for a long long time.

Oh, and you should find more tactful responses than the one above, that will not get you the kind of responses you are looking for, and is actually pretty offensive (and trust me, I am far from a "sensitive" person).