As mentioned in the threads shown above, the operating temperatures run from about 180 degrees to about 300 degrees. Measuring surface skin temperatures with a infrared thermometer can vary wildly from each measurement attempt. You have to pick one spot and use that as your reference point all the time. Surface skin temps are different from the temps on the inside. Generally, the most common measurement point is right under the glow plug, so you want to point your infrared thermometer right there at the base of the glow plug.

The surface of a engine varies a lot, from different color anodizing, different color paint, and whether it is shiny, or dull, or sandblasted gray in color. All those types of surfaces affect the temperature reading you get to measure. The best or more optimum method is to use a thermocouple, where you drill a hole in the head to place the thermocouple down inside the head at the high temperature position as close as possible. Since that isn't very practical, they usually use a washer size thermocouple and put it under the glow plug.

The reasons the temps can vary wildly, really depends on what the engine is being used for. A open engine exposed to the airflow on a sport plane being flown at 3/4 throttle will run a lot more cool than a enclosed cowled engine being used in a pylon race.

Methanol helps to cool the engine more than the lubricating oils. Nitromethane doesn't cool, so as the nitro percentage goes up, so does the heat. Also synthetic oils break down at high temperatures (they call it unzipping the molecules) an actually turn into more combustible products and instead of lubricating add to the engine's power output. Thus the peak in RPMs just before the engine seizes on you. Castor oil does eventually break down at high temperatures, but its breakdown point is higher than synthetics, also castor oil forms varnish like residues on the engine surface parts that help prevent engine seizing, at least for a little longer, but as the temps rise up, eventually it fails too. Also as the engine gets really hot, the internal parts expand fast and cause the piston to expand faster than the cylinder can. ABC engines had the odd trick of expanding and losing power when the engine got too hot and not seizing up right away. So sometimes you had a chance to back off the throttle before it locked up.

To really measure temps reasonably well, you need to use a thermocouple device and a data logger and track the engine temps while you fly, et cetera.
Now car people like to use the infrared thermometer to good effect. You run a lap or two, quickly stop and make the temperature reading, and repeat as needed.