Can I use or access the San Francisco font that comes with El Capitan?
You can download it from here: https://developer.apple.com/fonts/

But there is this note in the license agreement:


  IMPORTANT NOTE: THE APPLE SAN FRANCISCO FONT IS TO BE USED SOLELY FOR CREATING MOCK-UPS OF USER INTERFACES TO BE USED IN SOFTWARE PRODUCTS RUNNING ON APPLE’S iOS OR OS X OPERATING SYSTEMS, AS APPLICABLE.