I worked at a Harley shop fro awhile so I will try to help but may be a little rusty. Sportsters use and oil tank located uner the seat. There should be a dip stick right next to the seat usually on the left side if I remmember right. Under this tank there is a drain, may be a hose attached to it with a plug in it. Pull the dip stick, drain the tank. These are dry sump engines and have no true oil pan like a car. I recommend only harley oil filters and oil. Do not use synthetic and do not use car oil. Harley Oil or Motorcycle oild for harleys is designed to help deal with the cylinder head temps and oiling and cooling in a aircooled engine. It is not that much more expensive so do it right the first time. I should also tell you that for around 50 buckss you can go the your local Harley shop and purchase the service manual. It will tell you how to completly tear down and fix with many logic diagrams to help diagnose a problem. Follow the service list recommendation for the mileage on the bike. If you follow the proper service schedule you will have few if any problems. Most newer harleys do not leak. My 99 has never left a spot on the ground. It was also serviced properly from the day it was purchased. As for oil in the air cleaner, sounds like a after market cleaner. Some of these have a reputation for leaking. I would look to make sure all the hoses are connected properly and all the bolts tight at the air cleaner. Let me know if you need any more info.