Using Nitro in the 5-15% with at least 20% Oil won't affect the life of an engine, However when using higher %'s of nitro the engine life will be reduced especialy over 50% nitro. When I used to race boats in the 80's I used 60-70% nitro and 20-25% caster and the engines would only last a couple of hours where as the practice engines that I used on 10% nitro would last forever. After I stopped racing I went to 5% nitro for a while and then to 0% in all engines and when tuned correctly you have no problems with them.

Cheers