Motorcycle Questions? Ask a Mechanic.
There are two oils you are concerned with.
First is the engine oil. It is drained via a single rubber hose that is attached to the bottom portion of the rear of the frame, just in front of the rear tire. Warm the bike up to help the oil drain properly. Loosen the clamp at the end of the rubber hose and pull it off the plug and it will drain into your pan. Once drained you can spin off the old filter and spin on a new one. Refill with the oil of your choice. I recommend 20W-50 and it takes about 3.6 quarts with a filter.
The other oil is for the primary and transmission. There is a single drain plug located under the clutch cover near the rear of the primary. Refill through the clutch cover with the special Sportster Primary and Transmission. It takes 32 ounces.
Take care to inspect all o-rings to insure they are in good condition.
I hope this helps. Please let me know if I can be of further assistance. Also please let me know if your bike is very custom and does not use the stock oil tank or frame and, with a little more information, I can help you figure it out as well.