Engine manufacturers spend a lot of money in the certification process determining how their product should be operated and when matched with an airframe where the green arcs on instruments should be placed and any do not operate ranges due to engine/airframe vibration issues.
So clearly it is OK to operate in the "green" all day without causing grief.
As a general rule running low compression slow revving engines at upper power settings is better for them than running them soft, less chance of cylinder glazing and better ring pressure on cylinder walls as most aircraft engines have Keystone Compression Rings.
Heat is not the enemy, if the engine is producing power it will be generating heat, there are more problems induced by rapid throttle moves particularly from low power to high power than will ever be induced by so called "shock cooling".
Rapid movements introduce lots of cool fuel/air mix without allowing the cylinders to heat up and at the same time the extra stress on the engine leads to cylinder head cracking particularly around the sparkplug area.
Rule of thumb with big radials is if you can see your hand moving the throttle you are moving it too fast !!!
The major scource of engine wear and low life is the number of times it is heated and cooled (thermal cycles) running the engine for long periods at proper power settings is good for the engine.
Another urban myth to expunge is the one that says a good engine does not use oil, Engines should use oil, lubricates the upper cylinder.
It is the useage pattern that is important, yet many owners/operators don't record oil use, if they did it is one of the best indicators of engine condition along with cylinder leak down tests.