Jim the nitro content isn't critical but I would use 10-15 % on the .15 and add oil till you get 23-25 % at least for break in.  Later you may be OK with 20 but I use at least 23 on every engine I have and up to 29% on my old classic style motors,  of which I would consider most any Fox.  Break it in by running it moderately fast but cool with a smallish prop.  Maybe a 7x4.  Start in a fast four cycle for the first 15 minutes,  go to a 2/4 break for 15 then an easy 2 cycle for another 15 minutes.  It should then be ready to fly but set it a little rich for a few runs.  Put your flight prop on for the last few minutes on the stand.  I would guess that will be something in the 8 inch by 4-6" pitch range.  A mistake is often made by putting on too much prop and making the engine run too hot.

Dave