Bill, I'm asking this question on behalf of a friend... He went to his local hobby store and wanted to "downgrade" from his usual 10% nitro blend to one with 5%. I'm not sure what reason he had for wanting to do this apart from slight cost savings. Apparently, he was told by the salesperson at the store that its bad for the engine if you switch from a fuel with higher nitro content to one with lower nitro content. When he approached me, I couldn't see the logic behind this salesperson's rationale apart from trying to sell more expensive high nitro content fuels. I looked through the manual of my 46FX but there's no mention of this there apart from running through the break-in procedure whenever one changes the fuel. Is it safe for me to assume that there shouldn't be a problem for my friend's OS 91FX too?

Wtih a ringed engine, and using fuels with 15% or less nitro, you can pretty much run whatevery you want whenever. You'll have to reset the carb a bit to accomodate the fuel change, but not much else. As long as you have enough oil in the fuel, you'll be OK.

If you make a large change, such as from 10% nitro to 30% nitro, then you'll have to do a bit of breakin, because the engine will run a lot hotter and the parts fit will alter drastically at the hotter temps.