I decided after 4 seasons to change my spark plugs. I find no recommendation in my manuals, except to clean and inspect them at 300 hrs. They are platinum plugs so they may go longer. As you may know there are 16 spark plugs. The engine comes with Motorcraft CYFS 12NPG plugs which I cross referenced to NGK 6509. The manual says to gap at .044” but I found all the originals gapped at .040”.
The 8 plugs on the sides of the engine are in “conventional” positions. Take off the access covers and you can sit down in the ballast bag area to do those. Be careful to pull the wires off by the boots, not the wires. I used Silicone Dialectric grease in all boots. Make sure you feel a click on replacing the boots over the new plugs.
The 8 plugs on the top of the engine are easy except for a few parts to remove to get access. Starboard side has an electric panel and underlying bracket that must be removed to get to the coil packs. A couple zip ties also have to be cut and replaced. Port side has a relay panel to remove. After that there are 5/16 inch bolts holding the coil packs in place. Remove these and pull the boots off.
Since I had my bags out for winterization it was a good time to do this. Probably took me 1 1/2 hrs. All the original plugs looked pretty good so they probably could have gone a few more years. I think going forward I will replace them every 5 seasons or so.