Hello !
20W50 is definitely the way to go. You may also use monograde 40 or 50 weight oil if you can find a very good quality engine oil of this grade.
As per synthetics, versus mineral, there is a whole war on this subject. Half of BMW airheads user swear by the synthetics, the other half say it is terrible and leaks.
Whatever technology you choose, buy a very good quality oil and monitor the change in color as you use it. If it becomes cooked, time to put something better into the bike.
bear in mind that the 50 value here is the KEY factor. you want the oil pressure to stay high when the temperature goes very high. Think about the value the temperature gets after one hour ride at the main bearing under this outside temperature...