in the old days...
Sudden Winds,
You may be interested to know that in the piston airliner days of yesteryear, these aircraft were ALWAYS cruised at a constant power setting, regardless of weight.
Higher the weight, slower the speed, and v/v.
Take the DC6 for example, cruised at 1100 BHP, always...approximately 45% of (wet) rated power, with a fuel consumption of approximately 95 gal/hr (per engine).
As for cooling, that's what the cowl flaps (and where fitted, oil shutters) are for.
And as for 'cycling' power every so often, ah no, not done in these machine either.
The only time power was adjusted (in cruise) was when it was necessary to shift blower speeds...'round about 12,000 feet or so.
About the only time you would want to vary the RPM in cruise would be with a new engine/cylinders, as per the manufacturers instructions.
Flat (opposed) engines have the same combustion events occuring as radial engines of long ago and can successfully be operated lean of peak in cruise PROVIDED the fuel/air mixture has equal flow to each cylinder, an area in which the radial engines excelled.