The stator is the part where magnet wire is wrapped around the lamination stack. I haven't looked at the Busa one, but usually the magnets are on a shaft on the inside. As the magnets spin, they generate current. It is OK to have oil on it. The magnet wire has a little insulation on the OD and the stators are usually varnished after they are wound. The insulation and varnish will break down over time and you can get shorts from the magnet wire to the lam stack or from wire to wire. You will stop generating current and will need to replace it. Sometimes, you can get away with cleaning, baking, and revarnishing.