I run 5% nitro 18% oil (20/80 castor/synth) in my OS FS-52S.

I tried 0% nitro, and it worked well, but it was harder to adjust the idle mixture. But it was fine for sport flying. The fuel consumption is very low, but there is slightly less power than with 15% fuel. By going up to 5% nitro it idles smoother. I have settled on 5% because it runs smoothly but still costs a lot less than 15% here in Norway.

"I have heard" that OS engines are made for higher nitro than Laser engines. OS recommends 5-15% nitro for the FS-52S. To compensate for lower nitro, Laser engines have higher compression. My Irvine .53 MkIII runs perfectly on straight fuel, and it has a very high compression. So I guess it all depends on what fuel the engine was made for.

Tore S