Well, not so in spark plugs!  1994 GMC K1500 4wd, 350 TBI with 156,000 miles.  This set has an unknown amout of mileage on them, as they were in it at least from 139,000 when I bought it.  The only symptom of a problem was a rough idle.  Otherwise, it ran well, burning a quart every 3,000 and getting 17 mpg highway.  Note the #4 (second from bottom on left) had a larger piece of greyish deposit on the other side that fell off during handling.

I changed both plugs and wires on Saturday, and the idle is much smoother now .  I wonder how many more miles this thing has in it...

btt

The greyish deposit that fell off is ash from burnt oil.  I do a lot of spark plug analysis and quality control for my job, so I look at a bunch of plugs every week.  I'd say your plugs have at least 50,000 miles on them.  You can see the wear in the ground electrodes.

Always keep track of what plug went where. It will tell you a lot about that cylinder.

What's the brown deposit from?

Do I have any that are in 'normal' condition?

The brown is also ash from oil.  The color depends on the spark plug tip temperature which is different for every culinder.  I only see 2 plugs that look close to mormal, the top right and the second on the left.

Quoted: The brown is also ash from oil.  The color depends on the spark plug tip temperature which is different for every culinder.  I only see 2 plugs that look close to mormal, the top right and the second on the left.

Do you believe in the platinum type over the old style?   I was told the standard ones actually are slightly better on emissions.

What change interval do you follow?   I tend to change at least once a year -- they're cheap (at least the old style) and on my vehicle, easy.