Be prepared to be "nuked" as folks are kind of sensitive to redundant oil threads. I know there are many many oil threads on this forum. Google oil:fjrforum just gave me tons of hits.
So to make this simple I have used both 10W-40 and 20W-50 dino from our local Prosser, WA motorcycle shop. I just use Suzuki branded oil because it has been cheaper than the Yamaha oil. My only reason for changing to synthetic would be if one would be tired of changing dino every 4000 miles. I believe SkooterG who has over 200K on his FJR says dino is just fine.
Just put the right kind of oil in it between the two lines on the sight window and I believe there is lots of history saying the FJR is just fine. I think Victory even still makes the 20W-40 that the earlier Gen II manual says to use (that hardly any of us ever used) if you wanted to be really particular about it. I worry about figuring where to ride, rather than worrying about which oil to use.