Without the like of GAMIjectors fuel flows are more often than not uneven across your cylinders making lean of peak operation imprecise.
Yes, but uneven fuel flows make ROP operation equally imprecise.
And the crucial point (not understood by >90% of pilots) is that a cylinder operating 'slightly rich of peak' is in a much, much more critical operating regime than one run LOP. A mixture setting of around 50 degF ROP means you are forcing the engine to run with the highest internal cylinder pressures, the highest cylinder head and valve temperatures and the greatest chance of detonation. All of these adverse factors diminish as you weaken the mixture further through peak EGT to LOP.
All of the hard data (rather than 'marketing fluff') contained in aircraft piston engine manufacturers' manuals over many decades show this to be factual. Unfortunately, power diminishes as you weaken the mixture from 50 degF ROP, adversely affecting speed. I wonder why aircraft marketing departments don't like that? Equally, as you richen the mixture from 50 degF to 150 degF ROP, which is the minimum advisable for
any of the cylinders at say, 75% power (250 degF ROP for 100% power), fuel consumption increases significantly for almost no increase in power. Not surprisingly, the aircraft marketing departments don't like that either.
So, despite the hard data to the contrary from the engine manufacturers together with decades of experience from piston airliners, G.A. has ended up believing that engines should be operated at the worst possible mixture settings from the perspective of engine longevity.
For the larger engines at power settings above 65%,
if you run ROP you must run strongly ROP on all cylinders. To have any cylinders around 50 degF ROP is the worst place to be.
Kiltie, don't consider it rude if you wish to suggest that posters should ignore my advice ... just give them the technical reasons for doing so.
That engineers and other pilots alike advise operating the engine in the worst possible way isn't of much help. Precisely why do those people think that slightly ROP on one cylinder is a good place to be at high power settings? Try asking them to sketch a chart graphing EGT, CHT, ICP, HP and Valve Temp against mixture to see if they have any technical basis for their argument (or, indeed, any knowledge at all of the subject!). Then find some engine manuals (the earlier Lycoming and Continental manuals contained much more test-stand data than the later ones) to see what that chart really looks like. If you want the most comprehensive data, find a manual for one of the old Pratt & Whitney radials. These engines were traditionally run LOP yet, thermodynamically, they were just the same as our flat fours and sixes.