If the trucks are overheating, there is another reason for that, not oil related, they probably need heavier duty radiators, however, under extreme condition, Delo 400, Delvac Super/MX, Delvac-I would all do well, Delo 400 is the best value considering its Indonesia, under high temps, it will retain its lubricity unlike a regular SAE 40 oil, also leave way less deposits in the engine.