I'd like to get your learned opinions/experiences with regard to the two brands of iridium plugs we have on the market currently: NGK and Denso. Do you think or know if it is advisable to upgrade(?) to iridium plugs? If yes, which brand is better? Are there considerations for different brands of cars (Denso for Toyota, NGK for Honda, etc.)? If you "upgrade" to iridium plugs, when do you have to replace them again (every 10k, 20k)? Thanks for your inputs!!!