Hm, ok, I'll take you guys word for it, though I still don't understand how that affects the only two characteristics that matter to a nitro engine, longevity and performance. I'm still going to advise regular people (as opposed to serious racers) to break in with the same fuel they will run, as I don't think that it makes enough difference which fuel is used to break-in (from personal experience with a wide variety of engines and displacements) to be worth the cost and hassle to someone who will only be buying one engine every 3 years or so. Those that would benefit from running an engine so perfectly broken-in, probably don't need to be told this. I certainly never heard it in a long-ish on road nitro racing career.

Edit: oh, wait a minute...is this a recent development in fuels? If so that would explain it, its a good 5 years since I last broke in a nitro engine. If I'm simply out of date, I can accept that.