I'd recommend the 10% over the 15% for break-in. The 10% has a tendency to run cooler. I also like to add extra castor as an added safety factor for those accidental short bursts of over-leaned runs. This is the way I break an engine in. I'm of the mind-set that the engine's first breaths of life will determine how well it runs after break-in, so I treat it conservatively at first.