Fuel should be fine (If you look around, there's many discussions on which fuel formula to use, each diffrent from each other . I run 15% nitro with synthetic/castor mixed fuel, other run straight synthetic and like this better....(But yes, some added castor for break in is probably a great idea)

Your engine is serioulsy overspeeding at those high revs.
Normal speed range for a .52 and a 12X6 prop should be around 9000-11.000rpm, not much more. If you're seeing that high rpm (15k+) you need to restrict it by propping the engine harder = more resistance = lower rpm.
I run 12X6 props on my .52 fourstroke engines.


Try a 12X6 prop and go easy on that engine during break in.
After break in, slowly adjust the high speed needle to maximum rpm and then back off -500rpm from the top.
When flying, you should see a white smoke trail, indicating a nice, rich setting and the engine will go and go and go and go and....