The type of lubricant is part of the design criteria of driveline components. I use only the recommended OE lubricant for transmission, diffs and the transfer case for that reason. Better lubricants are not necessarily better for a component. Engine oil is an exception and I don't want to get into that either. As far as engine oil, any good brand with same specs and weight changed frequently will be fine.

I would be as or more concerned with the suspension, tires and brakes.. I would refresh the suspension and go to heavy duty shocks and struts if you haven't already. Tires? I don't do any towing with my X5 so don't know what is out there but I would want them on wider rims than stock. I would go to bigger brakes.

That said, I think you are asking for trouble using your X5 in the conditions you mentioned. You need a vehicle that is built with towing in mind which would include in the conditions you mention. Otherwise, my guess is you will be very late for some of your destinations.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas