Leaning the mixture is not necessarily, or even just, about producing best power, although it can be eg leaning for take-off at high density altitudes.
Best range or endurance is at least as important.
Financially, best economy by reducing fuel consumption is also a consideration.
Lean at all altitudes as long as the manufacturer's leaning parameters have been met.
Last edited by Tinstaafl; 19th April 2003 at 23:21.