Type specific leaning above 5000 ft
Hi
Although some of you are quite correctly stating what the piston engine manufacturers recommend, one must bear in mind that they wrote that advice for USA conditions.
In South Africa ( say Johannesburg altitude 5500 feet AMSL) most un-turbocharged/supercharged piston engines produce better power with a correct slight leaning procedure applied - before take-off.
When flying tubocharged engines use full rich mixture -as the engine behaves as if at sea-level. The only exception to that is when taking off at 10 000 feet, such as at Lesotho s Letseng airstrip.
There have been incidents in South Africa of "rich cuts" on final approach because the book states " mixture full rich", but the book doesn't know you are at say 7000 feet.
Use common sense & find out what works in such an area.
Many pilots never experience operations at such altitudes, so again use common sense and obtain guidance from SOP's.
Happy landings.