Think about it this way, there are a lot of BMW's, Triumphs, Harleys and bikes going back to the earliest times that were not water cooled. They cruised highways just fine.

There are actually three types of cooling, water, air and oil. A water cooled engine is designed with tighter tolerances on the basis that if something is a bit tight and the extra power from the same capacity makes it run a bit hotter the water will get rid of that heat in a much more controlled way. A pure air cooled engine (like a Enfield) is designed for the optimum temperatures, the tolerances close up to make better fits as it warms up, then get a bit too tight causing wear if you run it too hard for too long. This I'm guessing is the sort of motor people are telling you about. A Bullet motor with it's 1950's materials and ancient lubrication circuit won't run for tank after tank at motorway speeds without wearing faster than it should. The oil cooled version simply has an oil cooler in the circuit so the oil mimics some aspects of a watercooling circuit. It's nice and simple without extra pumps, pipes and other bits that can fail, but isn't quite as efficient.

My air-oil cooled Triumph which uses modern materials will cruise at 90 mph all day. My aircooled Bullet didn't like more than an hour at a time at a constant 55 mph. I don't know where the XR falls in terms of technology, but I suspect it's much more like the Triumph.

I'm sceptical about gauges. To me they only cause worry when the air is a little drier or hotter and are horribly innacurate (my last one measured drinkable tea at 110 degrees C!), but if you want something for comparison, they work for some people.

Andy