I use a P3 O.S plug but I would have P3 and P4. P3 is ultra hot and P4 is hot. I believe the P4 would be closer to the CTO6. One thing to understand is once you put an O.S plug in your novarossi engine you have to stay with O.S plugs. The O.S plug has a slightly different thread. So once you use and O.S plug the novarossi plugs will not work.