Many answers to that question! Many here use Shell Rotella T 15W-40 dino or Rotella T6 5W-40 synthetic. Another favorite is Mobil One Truck Diesel 5W40 synthetic. Just make sure the spec label does not say "energy conserving" (the clutch will slip.)
Ron in Oregon
AVA Webmaster ("master" is optimistic!)
AVA Board Member
1995 Voyager XII
FWIW, I use the Rotella T-6 in both my bikes. Change oil/filters at 5K on both and have had no problems in the little better than 90K miles I've put on them.
Charles Hairr
1997 Voyager XII
2016 Honda Goldwing