I had some questions about my humminbird fish finder/depth sounder. I called Humminbird and they were very friendly and answered all my questions. I also ordered some parts from them and they were reasonably priced and arrived promptly.
You can find their contact info, troubleshooting tips and a downloadable user manual for your unit here;
http://www.humminbird.com/productLines.asp?id=602
I wouldn’t hesitate to call their technical support people if you can’t sort out the problem after downloading and reading the user manual.
I’ve never heard of oiling a transducer lol.. but I know that if you use an inside the hull transducer they recommend a special epoxy.
Good luck!