in Germany and most parts of Europe usally a combined device detecting generated gas (large volume by arcing or small volume due to persistent problems) in a transfomer and low oil level is used. These device are called Buchholz-Relais.
In India there are several good manufacturers for magnetic oil level gauges.One source is Sukrut Udyog, 9/1A Erandawana Pune-411004,India.Email-sukrut@pn2.vsnl.net.in.He is also making oil flow meters and pressure release valves for transformers.
Web of Qualitrol,