What does the CR8 plug do vs the OE CR7?
The CR7 is a hotter plug. That means it can burn off a richer mixture. There is a formula = the best ratio of fuel/air, and I believe it is 14:1 (air/fuel).
Sometimes it is necessary to use a different heat range plug ie: winter/summer/worn rings/tight rings ect.
I have found using top quality oils (2t/4t) you can use a colder plug (BR9HSA) which is not sold in the USA. This plug burns cooler in the cylinder, and is considered as NOT acceptable in US vehicles but maintaining a 14:1 ratio, AND using top oils it actually will burn cooler, extend cylinder life, and meet emission standards in any state in the US.
The reason most people use a hotter plug is due to worn rings, and excessive oil in the combustion chamber. The #8 plug will foul under these conditions.