Many years ago I got chipped by a company training pilot for leaning in cruise at 1500' during a line check...he was most adamant that the aircraft manual, BN2-300, said no leaning below 3000'...as someone alluded to above...and he was WRONG.
What the BN2 flight manual said was do not lean in CLIMB below 3000'. And what that is based on is 75% power.
Takeoff and climb from sea level on an ISA day at MCP and you will be at 75% power at around 3000', where you should then lean on climb for best power unless you want to drown your engine in fuel and spend a lot longer climbing. Admittedly the average PPL will never notice the difference because he rarely climbs high enough...but believe me it makes a huge difference when climbing high on a hot day...we routinely climbed higher than 12000' at ISA+20 to pass through mountain passes between valley systems full of short, rough, steep bush strips at elevations of between 3500-8000' (ISA+20). We went for mths never operating at density altitudes low enough to see 24 inches MP on takeoff...let alone 29.6...and yes leaning on takeoff was as routine as scratching your balls while watching TV.
I own a normally aspirated 1970 A36 and it's fitted with an IO550B, Gamijectors and most importantly an EDM700...and Deakin and I are on first names, he's a terribly nice and knowledgable fella.
My POH has all the guff you see in your POHs about leaning for peak EGT (best economy) and 50 ROP(best pwr) and it's straight out BS.
These POHs are written by the airframe manufacturer not the engine manufacturer...they are written by humans who don't understand the science and are written to make the aeroplane look good....it's that simple.
The reason why 95% of engine don't make TBO is because they are operated via a combination of the POH, ignorance and legend.
It's an absolute fact that of you run an engine 50-80 degrees ROP you are operating at peak CHT..on my engine it's around 70 ROP...the EDM 700 has a download feature and I have downloaded a flight and graphed it as John does in his articles..in fact I will see if I can put them up here for you to see.
Ok here's some raw data....fuel flows down the left are actually in 1 liter/hr increments even though I have recorded it in GPH.
The figures at the very bottom eg 78LOP are how Lean of peak I was when I started to experience some roughness and terminated the flight test.
http://www.fototime.com/{5BB6EE5E-50...B}/picture.JPG
In the graphed version of the above raw data you see that all EGTs rise together, peak together and fall down the lean side of peak together. The CHTs are a little harder to pick in terms of where they actually peak....it's a long steady peak rather than a spike type peak...probably if I fiddled with the graph boundaries and squeezed it it might look better...cannot be bothered though. What it does do show what balanced injectors actually do
http://www.fototime.com/{77704DF6-76...5}/picture.JPG