Juse
Density altitude is what the engine responds to. When I was crop
dusting in Louisiana at near sea level with humid temps near 90, we
always leaned out to best power. Engines all went full TBO without
problems.
Here in Mobile, AL, we operate near sea level and as a general rule
will lean on the ground to avoid fouled plugs, and again when flying
above 1500'msl. If I'm doing high power operations I'll lean towards
the slightly rich side of lean for valve cooling. Otherwise I'll go for
best lean mixture. With fuel prices going where they are it makes good
sense to go as lean as you can safely but not to the detriment of the
engine.and overhaul costs. I've taken many high performance engines to
past the TBO with no excessive oil or fuel consumption by paying
attention to the temperatures and conditions.
Over 50 years of safe general aviation flying.
Ol Shy & Bashful