I read in my wife's 2000 RX 300 owner's manual that the spark plugs are supposed to last 100,000 miles. I can't believe anyone would leave spark plugs in an aluminum head for that long. It would seem to me that they would be seized to the the aluminum head by then and you'd have to replace the head. [ Maybe that's what they had in mind ] It's hard enough trying to get them out of a cast iron head at 40,000-50,000 miles. Maybe at 50,000 miles they should be removed and treated with anti-seize, inspected, and then re-installed if they're still good. Also what the heck is an IRIDIUM spark plug? How hard is it to change plugs in a 2000 RX 300? I still don't have a shop manual; I think it's extortion to have to pay the dealer $100.00 each for a 2-volume set of manuals but I guess it's coming! Guess I've been driving Chevy pickups too long!

 

I just bought the Bosch 4 electrode Plat Plugs today myself. Don't know much about Iridium plug but like you mentioned, spark plugs left in for 100k and an aluminum head = Helicoil possiblity. Search and read up, one gent has changed out his plugs without removing the intake assembly. Uses various elbow joints to get up under the intake. Looked at the engine today and it seems feasable. So will attempt soon.
Bryan

 

PS Using the Bosch 4 electrode Plat plugs in my 4Runner and it runs great!