The manual tells you to use 4 qts but don't do that. 3 1/2 qts is good but if you are not sure put in 3 and then add some as needed. The oil should be 1/2 way up on the dipstick this leaves room for the oil to expand when it is hot and it will help prevent oil from coming out the air filter.
The HD dealer used HD 20w50 syn 3 oil and it will mix fine with the left over oil that is in the bike.
I been using Mobil 1 V-Twin oil for years and I am happy with it.
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08 FLTR candy red sunglo,with 2015 103 HO, V&H Dresser Duals with Monster Rounds, Power Vision tuner, Willy Shiny Wheels and Forks and a bunch of other stuff.