S&S Recommends Conventional or Synthetic 20w50 VTWIN motor Oil for use in all their Engines.
Also, if you want to do some reading - here is a research study available online from AMSOIL very detailed and offers you a little technical information on each 20w50 tested.
http://assets.ngin.com/attachments/document/0016/3287/A_Study_of_Motorcycle_Oils.pdf
A few common customer preferred options for you to consider:
Mobil 1 Synthetic VTWIN 20w50
AMSOIL MCV Synthetic 20w50
Bel-Ray VTWIN Semi-Synthetic 20w50
Royal Purple MAX-CYCLE Synthetic VTWIN 20w50