I've ran as low as 5% in a SH .28 - the only appreciable difference between 5% and 20% is it wouldn't wheelie on 5%. To match the performance of 20% using 5% nitro the head button and squish band need to be machined down a tad to raise compression higher than removing the head shim alone. (I removed the head shim and used a MC59 plug and it ran okay.)

Most sport engines will run on just about any nitro amount, and like proanti1 said; Your engine wear will be no different on any fuel so long as the needles are adjusted properly.