You can run an aircooled engine on the motorway without any problems.
Think about it this way: When doing offroad work you usually have pretty high revs and comparativly low speeds, meaning little cooling of the engine.
On the motorway you also have high revs but lots and lots of cooling, so no worries there.
If you're really worried about that, there's also oil temperature gauges that replace for the normal cap of the oil tank, so you can have a look at the temperature while riding.
See ya!
buebo
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