Another poster already linked to 2 articles from the "Pelican's Perch" series.
Although this discussion is 4 years old, I'd like to add the 3rd link that acpilot probably wanted to post:
This article can be seen as a prequel to the other two, starting with the basics and later making the points for running LOP:
- Why leaning is important (safer engine operations, less fuel consumption)
- Basics of combustion and mixture
- Uneven mixture distribution is
- the cause of the "shaky engine" when you lean too much
- reducible only with fuel injection
- why you need an EGT/CHT monitor on all cylinders (see other answers here).
- The relation of power, fuel flow, EGT and CHT and how to interpret plots of those variables; this is where Deakin gets to the point that
- there are points of similar power but with less fuel consumption and cooler temperatures on the lean side of the EGT curve.
- the idea of "taking the throttle back a bit" right after takeoff on normally aspirated engines is a bad idea (CGT gets higher instead of lower)
- using an engine monitor, go LOP using the hottest cylinder as your reference.
For high power, maximum-performance operation, you should run richer
mixtures and higher RPMs. For low power, maximum-efficiency operation,
you should run leaner mixtures and lower RPMs.
Deakin answers OP's question implicitly in this article supposing engine manufacturers wanted to be on the safe side (emphasis by me):
It appears to me that running in accordance with the POH will provide
good results in the worst possible cases, and TCM probably felt this
was their best option.
This and the other two articles convinced me back in the day to fly LOP (on an IO 360 A1A, EDM 700 monitor, fuel flow meter). That gave me a smooth cruise with ~ 10% less fuel consumption.
FWIW, I read all of John Deakin's articles on AVweb and highly recommend them. They provide great insight from a aviation veteran - sometimes grudgy but mostly substantiated with sources as far back as the Wright Co.