BMW, Porsche, Mercedes Benz, all REQUIRE Syn or they will void engine warranties. Synthetic is cheap insurance and peace of mind IMO. What ever brand you choose is up to you. RP, AMS, Mobile 1, are all great. I run Redline SYN in my primary, and it seems to run more quiet and cool.
This, plus harder shifts into 1st and 2nd with a loud clunk is the case when i used the syn. I wouldnt say syn is bad but for air cooled bikes i wouldnt recommend it. When syn gets hot enough it becomes like water and thats not what i want in my bike but i know for a fact that it has the problems listed above.
You also have to think, sure EBR recommends asmoil but thats also for the 1190rs which is liquid cooled.
Non-synthetic and semi-synthetic oils are better than 100% synthetic for very large or very old engines with low tolerances (how precisely the parts were made). The reason is because conventional oil has MUCH longer molecule strands and is more tangled up. The conventional oils are not able to seep into some of the small areas like synthetic. Synthetic oils on a low tolerance engine will cause the engine to eat oil, whereas using non-synthetic oil will often prevent the engine from eating oil.
Most engines built nowadays are done with extremely high precision so most of the time synthetic is the way to go.
Well syn will beak down slower at a higher temp, so if the syn is turning water like then the conventional would be water like even faster/at a lower temp.
I will admit I've heard a lot of people complain about noise though.