RE: Is 5% fuel OK to use?
Most engines work well on 5% Nitro fuel, European engines are designed to work on 0% Nitro.
Studies have been done to show the effect of higher nitro blends on performance, Yes more nitro means higher RPMs - but once you pass the 5% mark the increase in performance is marginal.
Basically if you are competing and every ounce of speed is important - Go for the extra nitro!
If you are just a Sport flier that likes to bore a hole in the sky, 5% will work well for you. You'll just have to tweek the needle settings a bit.
Of Greater concern should be the amount of Castor Oil in the blend - regardless what engine you have you need the oil to keep the engine cool and lubricated. (there are lots of threads on that subject!)
Happy Landings!