Not too much, just more than necessary. I've tried both and find that I couldn't really tell a significant difference between them in OS engines. I'd go with the lower cost of the 10%. The castor thing is more important, it gives you additional protection against inadvertant lean runs. I run 15% nitro in my 4-strokes, but I still run Omega fuel, it has about 5% castor in it. Might make my muffler dark, but I can deal with that!