@Ionic
Ans1. Yes they do have effect on CNG Average. Use Bosch WR78 4 tip Plugs i m using them for almost 6 months, my car's avg and performance is very good so far.
Ans2. when u use Bosch WR78 u dont' need to gap them as they r pre-gapped.
Ans3. Bosch recommends to replace these spark plugs after 30,000 Kms
This is what i've experienced and found them good and reliable but u may also look for NGK Platinums.