5% nitro! Did you forget a number there? If not, 20-30 is a much better choice of fuel... anyways, with those usually the hottest or next to the hottest plug in a company lineup is usually the best choice.
Your engine will run MUCH better and be easier to tune if you run 20%.
Even 10% can difficult to tune when the weather is cool. As for Motor life, I doubt that you will see a noticable difference with 20% or less.