Thanks for the infoThe schedule from the manual has you replace them like every 12k miles which is ridiculous. If yours are 21 years old I would say yes replace them, if you have put those miles on in the last year?
Pick a number between 3-5 yrs and one between 30k-50k miles and use it for your OEM plug changes, just do it on a consistent basis. And it never hurts to pull them in between to check for running condition and regapping.
The OEM IIRC does have a platinum tip.