This is the first report I have read about an oil spot on an X-Series camera.
An oil spot requires wet cleaning. As mentioned above, proper technique must be used.
If you have great eye-hand coordination, work patiently and carefully, and have a proper cleaning kit, you can do this yourself. Otherwise use a reputable camera shop.
In my case the only practical option is finding a good technician. I had my Nikon sensors cleaned every six months. I have never seen sensor debris I could not remove with a Goto Rocket on the X-Pro 1 or two X-T1 bodies.