Spark plugs

[Blue)(Fusion]

I replaced my spark plugs today for the first time in a year and a half, about 15,000 miles.

My old plugs were stock Motorcraft plugs. Stock heat range, single platinum, but I forget the part number.

Anyway, when I pulled them, all of them were white, some had some white/yellow/brown deposits on them. Does this indicate the engine is running too lean? What should I do about that?

I clean the MAF every 2-3 oil changes. Air filter is clean. Fuel filter changed last oil change.

I have almost 112,000 miles on the odometer and I'm still running the factory installed HO2 sensors. Should I replace these? I have new Motorcraft HO2 sensors sitting in my cabinet, but was told it's not worth replacing unless the ECU throws a code relating to them, which it has not.

Would the mixture likely be too lean at all RPM ranges or just WOT when the HO2 input is not used? I modified my tune to add an extra 4% of fuel at WOT vs. what it has been doing. I never experience any detonation under hard throttle, but I do have a slight hesitation with light throttle at light load. Maybe this is due to the mixture being too lean at that specific power setting? It's been doing this for months.

Pictures of the plugs I removed are located here:
http://richgannon.net/gallery/thumbnails.php?album=26

I replaced the plugs with Autolite APP103's, one heat range colder than stock, and double platinum. Gapped to .051" and the same problem at light load persists.