We always use and recommend Kendall 20w50.
10w30
HD makes a weight of oil they call "Sport-Trans" that is made for use in the Sportster primary. Or, you can use any 30W non-detergent oil.
Mobil 1 is a name brand, what you need to know is the viscosity. Make sure your oil says 20w50 on the bottle. That is what most people use in their Harley.
20/50
20/50
The viscosity preferred by many people is 10 W, but you can also use 5 W or 15 W, according to how stiff you want the front shocks to be.
3 U.S. quarts
Of course you can but when you do you are changing the viscosity of the oil. Your engine requires a certain weight or viscosity of oil. Use only the weight oil recommended by the manufacture.
SAE 20W-50 Synthetic Motorcycle Oil
5w30
I understand you can use the same oil viscosity in both motor & transmission on the 2008 harley roadking!
Always use the Honda brand oil. 10-40.