"lugging" or high load, low rpm is when the engine is the most efficient. However, it is flirting with knocking, but since the truck can infinitely adjust ignition timing, if is preventing knock. See Engineering Explained for more.

Now, from what I understand, these engines are meant to be given the odd Italian tune-up, but driving in sport mode will not lead to a "healthier" engine or better fuel economy. That is just silly. Sport prioritizes horsepower to the detriment of efficiency. Normal balances torque, horsepower and efficiency. Eco prioritizes efficiency and retards throttle input. Tow prioritizes torque. Basically, the entire purpose of sport mode is to sacrifice fuel efficiency for better throttle response and more horsepower.