There is a check value that is supposed to stop the oil from the oil tank running down into the crankcase. Very common for them to leak, but not a big deal. Just change the oil and add the correct amount of oil and you will be fine. Always check the oil level when hot as the oil will be in the tank, but as it sits it will drain back into the case. What usually happens is someone not familiar with this check valve issue will check the oil cold and find it low and will add oil which then overfills it. They run the bike and have oil gush out all over the back tire creating a very dangerous handling issue. A member on this board does make and sell the check valve in case you ever want to change it out.