I understand you can use the same oil viscosity in both motor & transmission on the 2008 harley roadking!
I am not sure of viscosity, but if you have Harley Davidson on your fuel tank, you should use Harley Davidson lubricants. H. D. primary and trans lube or H.D. 20/50 synthetic motor oil will work just fine.
You can use the same oil that you use in your motor
Usually it's called transmission oil. For example I use red line transmission oil in both my motorcycles(Harley,Ducati)
20-50w
NO
It would eventually take a toll on the transmission. ATF and hydraulic oil have different compositions, detergents, and viscosity ratings.
no...but you can use screaming eagle synthetic oil in both the motor and the transmission which is supposed to be better anyways
(NOTE: this answer covers both engine and primary/transmission lubricants, since the question did not specify which was of interest.) The recommended engine oil for this bike is Harley-Davidson's proprietary "Genuine" brand engine oil. However, many aftermarket oils are available. If you use an aftermarket oil, make sure it is labeled "for use in Harley-Davidson motorcycles" to make sure additives, temperature tolerance, etc., comply with Harley requirements. Automotive motor oils are NOT recommended for Harley engines. In a pinch, if you need oil NOW and can't find a Harley shop (or a shop selling aftermarket "for Harley" oil), the manual recommends using the proper weight motor oil specified for use in DIESEL engines. Recommended viscosity is 20W50 for all normal conditions. Oil capacity for the rigid-mount Evolution Sportster engine (1986-2003, 883, 1100 and 1200cc) is 3 quarts. The recommended primary/transmission oil for this bike is H-D "Sport-Trans" fluid, which is a specialized lube formulated to work with the wet clutch. H-D discontinued this fluid, and it is now superseded by "Formula+" fluid, which is a "universal" fluid for transmission and primary drives in both Sportsters and Big Twins. Many aftermarket primary/transmission oils are also available; if you use one, be sure it's labeled for use in the Sportster transmission, as the lubricant for the Big Twin transmission may not work properly with the wet clutch. Transmission capacity is about 0.8 quarts, but varies from one bike to another.
80W 90 if using conventional. If using synthetic, 20W 50 will work. Also harley has Formula +.
yes-absolutely.
80/90 semisynthetic hypoid gear oil is what the factory put in it but I use Red Line Heavy ShockProof fully synthetic gear oil. Not cheap but neither was your Harley.