I heard that G35 spark plug should last 100,000 mile. A mechanic told me that I should replace it every 30k mile. WTF?

What about other Japanese car? Civic?

 

Depends on plug. The older copper plugs recommended 30K change intervals and they are cheap ~$1-$2. Then they started getting into platinum plugs and then double platinum which then cost quite a bit more, but wear a lot longer. I think the next big thing is iridium, but I'm not sure if our cars have these. 30K is way too early in any late model cars so it's up to you if you want to change them earlier than the 100k interval. My '02 Odyssey has 105k interval.

 

Pshh i would check your plugs anyways. I just did mine at 69k and they had been replaced before with ngk's. The vavle covers had the leak problem and it was fouling the plug up. It cant hurt to take them out if the plug is worn replace it. I wouldnt wait until the last minute though with the problems the vq engines have been having.

 

I would recommend every 50-60K. Keep optimal performance.

The manual states 105K intervals.