I went to my local Toyota dealer to purchase spark plugs for my 2002
Wow! I've never heard of a spark plug that could go that long. I guess I don't mind paying $15 for each plug when they only have to be replaced every 4-5 years!
1) leaving spark plugs in THAT long runs a high risk of the plugs seizing in the head. Remove them slowly and use a little WD40. 2) another area of spark plug failure is the sealing compound between the porcelain and the steel shell. Evidence of combustion leakage will be dark brown "jet" marks on the porcelain where the porcelain meets the metal casing.
The factory normally recommends to change them at 100k miles. I suggest changing them at 80k for optimal performance.
In addition to longer life, iridium plugs normally have a performance benefit too. This is mostly negligible on normally aspirated cars. But, forcefed engines have proven to gain a few horsepowers and a few pound-feet of torque with iridium plugs.
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