Sometimes I just change them every year because the car sits and I know the valve seals leak some. Put a 75 shot of nitrous on there and that will clean them up.lol Maybe when they are new lol. I have run the UR5 Right now I am using a YR 5 NGK plug for the nitrous and I am not having any trouble using this on the motor. So if you are at 10.5 comp on your 350 you can use this plug. This plug is not extended like the UR5 plug so it will run a little cooler. If you think you may be too cold you can go back to the # 4 plug I think that is hotter in the NGK number system if not mistaken. The YR style plug may work good too. Here is a pic of that plug
