Boiling point is reduced at high altitude because of lower air pressure (about 92C at 9000 ft) Diesels also build a lot of heat when they are laboring uphill. If you’re going to drive in those conditions often, the best defense is a higher pressure radiator cap. This is yet another example of water NOT cooling better. The lower vapor pressure of water makes boil over more likely. So stick with 50-50, or you can even increase the glycol to 60 %.