Thanks to Tom Taylor for this:

Recommendation/Instructions:
The following information was originally sent to all General Motors dealers as a DCS message on October 14, 1999:

"It has come to our attention that some GM dealers sell a customer service to remove Platinum tipped spark plugs and clean the threads at regular intervals to prevent the seizure of the spark plugs in the cylinder heads at high mileage.

Platinum tipped spark plugs are designed to operate under normal vehicle operating conditions for up to 100,000 miles (160,000 kms) without periodic maintenance. When no engine performance concerns are present, platinum tipped spark plugs should not be removed for periodic inspection and cleaning of threads, doing so would compromise the spark plugs ability to withstand their corrosive environment. The threaded area, although not sealed, serves as a protective environment against most harmful elements. Removing and cleaning spark plugs will introduce metallic debris and brush scrapings into the thread area which may further the corrosion process.

Chromate coated spark plugs should not be wire brushed or handled in any way once they are put in service. Chromium topcoats form a protective oxide on spark plugs that is not effective if scratched. Both coated and uncoated spark plugs will have the best chance of surviving a corrosive environment if they are left in position. Attempts to maintain spark plugs by removing them and cleaning the threads can actually create the corrosive condition that the procedure was intended to prevent."

I wonder how that applies to Fords?

Several years ago I did a "routine" spark plug check on my 4.0 liter V6 in my Ranger... took me 2 days!

It had about 50,000 miles on it. Ford says the plugs are good for 100,000 but you know, I just wanted to look at em. They are platinum electrode Motorcraft plugs.

The problem was not corrosion of the threads but corrosion of the plug body. When I went to remove one of them it sheared off at the threads, leaving the threaded part in the head.

Many hours later (after removing the AC compressor and the power brake booster) I drilled out the center of the stump (in stages) until I had almost nothing left, then chased out the threads with a tap and some dental picks. What a PITA!

Since then I have always checked plus at 50,000 mile intervals. (I now have 170,000 miles on that Ranger)

Airplane plugs are steel plugs threaded into aluminum heads. There have been many problems with threads seizing in the head. Lycoming requires an anti-seize material on the threads before re-installation. I have used this same compound on auto plug threads since my Ranger experience. Never had any more trouble.

Evidently GM plugs might need the same treatment?

I had a Cessna 172 with a Continental engine and it was also suggested using an anti-seize grease on the plugs.
Once owned a Honda 600AN two door sedan which had a 600cc motorcycle engine, it required the anti-seize grease (a paste that looks like thick silver toothpaste). Anyway it's great stuff.

Miss that little Honda car, it got around 50mpg.

Continental used to say use a white lead based anti seize compound. I still use a can of white lead pigment I bought decades ago, with a little pickling oil mixed in it. Never had a seizure, not even on exhaust bolts or muffler clamps.