I'm running K&B outboards, 3.5 and 6.5. I use 15% Cool Power in both with good success and often use 30-35% heli fuel in the .21's. The one time I used a 65% nitro boat fuel it was low on oil. I found this out when I found two engines had broken rods. I'll stick to heli fuel and if I'm a little slower, well, ok.
Rick H.