You originally said 'oil' so I assumed that was what you meant. If it is fuel/oil, then this could be any of a number of things. If it is running good, then I would not suspect a leak in the primary fuel line because it would tend ot run lean. Others suggested bearings, and so on.. but they also said some oil/fuel is normal.

My new OS46ASX leaked like hell at the muffler pressure tube but the engine ran fine. It ended up being a loose through bolt that holds the muffler halves together. Another engine, a Super Tigre 75 leaked like hell at the engine/muffler bolt points. That was a matter of tightening the bolts that hold the muffler to the engine. Also, the oil was sort of brown in color indicating that it had gone through the normal fuel/oil burn cycle and was a normal exhaust residue.

Some oil is normal, some fuel is normal. But as someone else suggested, if it ain't broke, don't fix it.