Last week I reached 35k on the odometer. Besides changing the oil I thought it prudent to change the spark plugs too. I tow a fifth wheel camper and half the miles were done towing. Taking the old plugs out I saw that the engine seems to run a bit rich. The insulators were a medium to dark brown side to side and I'm assuming the darker side was the intake valve side. There were no deposits built up and all the electrode gaps were at .034" per spec and same as the ones I installed.
It's been a long time since I've read spark plugs on a modern fuel injected engine this dark. Truth be told it's been a few years since I've changed spark plugs. An '05 Lincoln LS that was prone to failed COPs and an '11 Nissan Frontier as it approached 80k miles were the last. If I recall correctly, the plugs were a much lighter color almost chalky white on those.

 

I run them @ .030 but ran them @ .028 on my old truck with a tune.

 

I saw a "recommendation" to gap them @ .030 - .033" but since the factory gap was close and set I left well enough alone. I'm sure the original plugs could have gone on much further. I've run nearly two thousand miles since with 500 towing and I've not noticed any change in performance or mileage. I monitor with Torque Pro app and use FORscan to read DTCs with no indication of misfires or trouble.

 

My old truck came with Motorcraft SP550's When I changed them the replacement SP550's were pre-gapped @ .030

 

Optimum performance = SP gap @ .030