why doesn't whoever writes the POHs advocate LOP
Some do, the Chieftain for example, yet if you read the Lycoming manual for the engine, it makes no mention of LOP. This of lack of consistency only adds to the confusion among some of we plebs.
The IGO and IGSO-540 was likewise approved for LOP. From the manual
"Lean the mixture until EGT peaks and continue to lean until the EGT drops 25 to 50 degrees on the gauge. Flying on the lean side is permissible if extended range and cooler engines are desired. Operation at peak EGT is only recommended for mixture control adjustments or when induction icing occurs. The Amount of temperature drop can be determined by resultant fuel consumption and engine smoothness.
"When operating on the lean side of the power curve, the pilot may observe that airspeed and power are less. If you desire to regain lost airspeed and continue to fly on the lean side of the curve, two steps are important. If sufficient throttle is available at the lower altitudes; first add two inches of manifold pressure to the standard cruise setting and then lean 25 to 50 degrees, (lean of peak). Occasionally, some pilots prefer to fly on the rich side of the power curve; this is permissible. Adjust the mixture control until EGT peaks and then enrich mixture until you get 25 to 50 degree drop on the EGT gauge. Acceptable continuous (cylinder) head temperature is an important reference here."
yr right, I might ask, if LOP is bad, bad, why then do Lycoming, in this case, approve LOP on these particular engines?