Depends how often you change it.i use syn. Only ,others use oil.its like your cage,if you tend to go past your regular change ,use syn.it takes longer to loose it's viscosity.there are many threads on this subject already.imo.im not going to use oil to save a few bucks,same in the octane debate,as long as i can afford the best,thats what i get.
When I put Amsoil in all three holes on the 95flstn, sold but missed, it was like a different bike shifting wise plus she seemed to be smiling and just once I thought she dipped her left blinker in a wave too.
i just changed to Amsoil and really like it. The only reason i went with this brand it is very easy for me to get. I have about 4 dealers within 15 miles.
Huge Amsoil fan here, I have ran it before and will be swapping to it at my 1st oil change (in like a week).
Syn is great! Amsoil, its unreal, a true difference, smoothness, shifting, temp, everything! Totally worth the money!