There is no one "best" oil, as there are way too many variables to consider.
Grab your owner's manual, there should be a chart in it listing several grades of oil as acceptable for use based on temperature range - use that to choose an appropriate grade of oil, and then consider a "high" mileage offering from whatever brand you favor. The "high mileage" oil offerings are formulated with a different additive package, typically higher levels of detergent and also products that help to keep oil seals flexible and leak free.
You mention steep hills, very hot places and deep sand - if the vehicle has an automatic transmission, consider fitting an auxiliary transmission cooler, and fit it in the transmission outlet line to the heat exchanger in the radiator bottom tank so that it's between the transmission and the radiator - you'll be generating a significant amount of heat in the torque converter and with the auxiliary cooler first, you'll be dumping that heat into the atmosphere rather than into the vehicle cooling system, thereby reducing the heat load on the cooling system, you'll also still have the original heat exchanger in the radiator to warm the fluid on a cold day.