I agree with downunder. I use cool power and add benol to it for all my engines. I like 15%.
I am breaking in a os91fx now and so far I am very happy with it. I have ran 4 tanks of fuel thru it (on the ground) but the ring hasn't set itself yet. I expect it will take a few more tanks and then it should be ready for the unloading of flight!
I try to run in the engine slow. I add some break-in time beyond what the manual says just for piece of mind and I can tell the Ring isn't set yet by the lack of compression the engine has when I roll it over by hand.
Use the 15% with the Klotz. It isn't the nitro that makes the difference on break-in, It's the luberication and the process of richening and to a point, leaning the engine. I like to use the same fuel I plan to fly on. 15%nitro- (aprox)20% oil and the oil mix should include Castor.