That's why you break an engine in a little rich. By running it a little on the rich side, you are giving the engine the extra oil it needs for break-in. Just break it in with whatever fuel you plan to use. I've been running 40% for the past 2 years and don't change fuels for break-in, and have never had a problem. (The only engine that did go bad on me, was partly due to a crack in the exhaust, making tuning difficult. Even then, there was quite a bit of runtime on that engine.)