In some applications the signs of overheating are just signs of the harsh environment the machine is running in and are unavoidable. I take it you are opreating this diesel generator in a very hot environment. We have had a lot of experience with diagnosing samples such as these and we tend to find a high percentage of the engines show signs of overheating. We have put this down to the application and environment they are operating in. The cooling systems are only able to cool to a certain degree and therefore the oil seems to take more of a beating than in cooler climates. The only solution would be to improve the cooling system, which is not always easy, or reduce the oil change/drain intervals to prevent the effects of the high temperatures reached from degrading the oil to a point of it not doing it's job.