I recently purchased a 08 King Ranch with a V10, 150k miles. Started having MisFires in three cylinders around 152k and I've read all the threads and possible causes, but here's my dilemma:

According to what I've read the (SP-509) Plugs work with the (DG521) Brown Boot Coils & the (SP-546) are used with (DG511) Black Boots. I get under the hood and find the following.

1. Someone installed some cheap Chinese knock off coils. (Xaexheart, you can buy all 10 for under $100 on-line) They look like Accel coils with black boots.
2. I pulled a couple of plugs and they're Motocraft SP-509's which still have the proper gap but you can see a Carbon Track on them

I couldn't find anything definitive that states which Coil & Plug work in an 08, so based on the plugs I'd purchased a new 509 plug with a DG521 Coil (Brown boot), but the Coil has some wiggle room after I tighten it down. Now I'm confused.

• Is it normal, for a Super Duty coil to have some play, I wouldn't think so? I'm assuming it's because the Brown Boots are too long.
• If I should be using the Black Boot Coils then do I need to change all the plugs to the 546 or are the Spark Plugs interchangeable? I was able to remove the two plugs easy enough but I'm concerned if the threads were damaged by installing the wrong plugs.

Any help or suggestions will be deeply appreciated