Hello and welcome! I suggest you search the forum for "engine oil", "transmission oil". etc... You will find tons of opinions. That being said I have a 2012 FLSTC and use:
Mobile 1 V-twin in the engine
Spectro 6-speed in the tranny
H-D formula+ in the primary
So far, no problems for me and I can tell you that the transmission shifts smoother now and is a lot quiter.
I think quite a few people on here use this combo. Amsoil and Redline are also popular.