We run (perhaps ran might be more accurate) large recip aircraft engines at
'10% lean from best power' (by manually
leaning them during cruise) for many thousands of hours and they worked fine
in that condition, matter of fact they'll continue to run fine as much as about
30% lean before they get unstable, they seem to love lean mixtures!..

Been there. Done that (R3350s). One problem is that the power curve is quite
steep on the lean side -- pilot has to be more careful. Never did it but have
been told if one can see the exhaust stacks, it is easier at night. One leans
until one gets the shape/color of exhaust flame that is "correct." Beyond me.

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