Originally Posted by
TelsBoy
There's nothing I could really add that's already been said here, most manufacturers recommend leaning above a certain density altitude (note not indicated altitude!) although this is often to prevent ham-fisted pilots from running the engine too lean and causing damage. 3000' or 5000' is usually the quoted figure in my experience.
Rick Durden writes about this quite a bit in "The Thinking Pilot's Flight Manual" (Vol. 1) and says that when the POH says this it is (almost?*) always referring to the takeoff or climb section of the checklist. The cruise section gives figures for a leaned engine and (again, almost?) always does not give a minimum altitude.
So,
in the cruise, lean at any altitude unless the POH explicitly says otherwise, and be sure that you're using the POH material that you've read in the intended flight regime.
Durden's is a good book - I am just about to start on the companion volume.
*I've written "(almost?)" in brackets since, as Durden says, it's what is in the POH that counts - not what is in any other document, and generic statements are often wrong for your particular situation.