I have no idea. I don't know all the technical details of the thermometer yet, but where on the engine should I be measuring?
Just as background on this type of thermometer -- there will be a visible red LED laser (pointer type) aiming dot which denotes the measurement location. Don't get the idea that the thermometer is simply taking the temperature at that point, however. The thermometer measures IR light intensity, which is a function of temperature. The way that this is accomplished is via a small lens assembly and an IR sensor.
The nature of this approach is that the farther you get from the thermometer, the more the indicated measurement will be an average of a larger area.
For example, you could put an ice cube on a tile floor, and point the thermometer at it -- centering the little red dot right on the ice cube. Depending on how far above the floor you are, you may get readings from 32'F to 60'F. It simply depends on how much of the IR from the ice cube is captured versus the IR from the background, and I stress again that this is dependent on distance. Your thermometer will come with a chart showing you the IR "capture circle" as a function of distance.
I tell you this because when you are measuring items which have a large temperature gradient, you HAVE to denote what the measurement distance was and what the IR window aperture (aka "capture circle") was. Otherwise, there is no way for someone to duplicate your results.
For comparison, let's look at a common application of these thermometers, which is to determine where insulation might be needed in a house. You point the thermometer at a wall or ceiling, and compare measurements with other locations. Now then, even at tens of feet distance, *most* of the measured area will have roughly the same temperature (say, 65'F). So this measurement is useful and can be easily compared to other measurements made by other folks using different thermometers.
In contrast, suppose you are pointing the IR thermometer at an engine, at a distance of 4 feet. The IR "capture circle" at that range may be 10" in diameter. So, for example, within that window is a plastic shroud at 100'F, an exhaust manifold at 700'F, a portion of the cylinder head at 200'F, the starter motor at 150'F, and a whole bunch of other parts at wildly varying temperatures. The IR thermometer will average the readings based on area (larger areas are weighted more heavily, of course). Despite the little red dot being on a small section of the engine block, you have a whole bunch of other temperature data intermixed with it. You didn't really measure the engine block temperature in this case.
If someone else tries to repeat this measurement at 4 feet, but instead uses a thermometer with a IR capture circle of 4" at that distance, they will likely end up with a much different value, depending on what is in the IR field of view of that specific thermometer.
I attach below an example I found via Google images; note the distance-vs-spot size graphic on the left hand side of the instruction sheet. This specification is good for this model only; other IR thermometers will have different specifications for the IR capture circle.
Wrooster