I am using 15%. I have always been told by most to use the 5%, but they have always done fine. The rossi temp is most of the time low on the 15% except with a long pipe. please do not take this the wrong way, but why use the low Nitro. Is in because of dead sticks. I know that the Moki bigger engine will not run well at all with anything over 5%. I have some 10% I use with a 1/4 stick and a ST 2300. I may need to use it for a while.