Reading from the pan will tell you the overall delta of the fluid. Reading from the hot line does not allow this as it reads from the operating position (max temp) of the fluid. I would suggest that if this is such a huge concern you install 2 gauges. 1 on the pan to measure what the fluid (in quantity) is doing - this will tell you how quickly it is breaking down and how soon it must be replaced and measure another reading from the hot line, telling you the max temp at the active area.

All of that said, this should only be a major concern for those pulling 12,000 + pounds up mountain passes in the middle of the summer! Remember, all diesels, regardless of brand, badge or loyalties are designed to work - al beit some better than others but work none the less.

My 2 cents