Some submersible turbines have oil filled bearings. They are fed with a tube from the top kind of like your describing. I have only seen one of these and the inside of the well had many feet of oil floating on top of the water. Needless to say, these things do leak oil.
Sounds like that's what you have and if you don't keep adding oil, the pump bearings all the way down will start to wear out quickly.
I had to hire a guy with a camera to map this 8" well. The poor guys truck was dripping with oil when he pulled the camera back out of the well. It was over 600 feet deep and the cable the camera hung on was covered with oil as it went back on the spool in the back of his box truck. The entire floor of his truck was oozing with oil. I suspect he spent the rest of the week at the car wash.