Mark, that reasoning is what's normally used when talking about running in an ABC engine but in fact it's quite wrong and I've never seen it in any manufacturer's instructions. If you try running in a ringed engine that way then there's a very good chance it'll be ruined. Tests have shown that varying the nitro percentage only makes a small change in running temperatures (on the order of only a few degrees) and high nitro can even make them run a little cooler.