My 1979 BMW R45 airhead specs 20W50 but it runs the oil quite cool which is not surprising given it has almost the same cooling fin area as other models with double the HP. I started with 20W50 then 15w40, 10W40 and for 18 years it's run on a quality 5W40. The oil designation is not the only criteria, I consider the actually viscosity of the oil at the temperature it runs at and I'm more than happy that my 5W40 @ 85 Deg C is a similar viscosity to a 20W50 @ 100 Deg C. Bear in mind that due to lower losses, a thinner oil will run cooler than a thicker oil so the viscosity drop is not as big as you might otherwise expect. If you are going to run a thinner grade also look for as high a HTHS as available in that grade.
Ambient temperatures sound to be a good deal higher where you are so I'd measure the oil temperature after a long run before making a decision.