I use Mobil 1 20W/50 for the engine oil and primary. The 75W/90 in the tranny of the Roadking. With the newer 6 speeds a lot are switching over to the 70W/140. Here in New England that's a little to thick for our temps.Harley wants you to use their tranny oil 99892-84 which is made by the lowest bidder it's either Delray or Sunoco then they mark it way up and tell you it's the best oil since sliced bread,in any case (pardon the pun)
Mobil 70W/90 is what most are using as an alternative .Which is also what Triumph recommends for the Final Drive in the Rocket.