I have a '99 2.5 l opti and the NGK iridium plugs are the pricey ones that are recommended as "heavy duty". For the 2.5 l block IZFR5G is the model plug to use but I am not sure if the 3.0 l block uses the same model. They range from $12 to $20 each or so. The last batch I bought I picked up off ebay several years back for much less...

kc