Oil heaters produce no fumes or flames and feature whisper-quiet operation. Because the oil within the heater never needs to be replaced, oil-filled heaters are maintenance-free. In addition, these types of heaters are compact and portable - allowing you to carry them from room to room. Many also feature carrying handles or a set of casters for easy mobility
some of these types of heaters have a hole with a plug, that allow you to add oil, but few of these are in use today. so this being said it is most likely the heating element is failing to fully heat up the oil. parts may be found, if researched in detail on the net


also be known that the electrical componets are most likely to fail befor the oil evaporates. so look at all overheating controls and so on.

good luck,