Most likely the air/fuel mixture is the culprit. Your engine might be running too lean or too rich which will result in it overheating. Take it to a proper mechanic and tell him to set the mixture right, or if you have access to a service manual, try to do it yourself. Hopefully, this will fix it. People often change the mixture to improve mileage, but this actually costs them more, as then the engine overheats and they have to change the oil earlier, and also heavily reduces their engine life. Try checking the honda atlas website, they might have the service manual, or maybe the owner's manual itself lists the proper ratio. Don't try to tamper with the air-fuel mixture in air-cooled engines. You might get away with it in liquid-cooled ones, but air-cooled engines already run 10-20 C hotter than liquid-cooled ones, changing the settings to an improper ratio will surely cause them to overheat. You'll end up changing the oil in less than 1000km. With proper settings, the oil should last 2500km easily. Then you can change it.
