Thank you Mr. Brandan and Mr. Adil for your valuable inputs.
Eventhough there was an over heating episode few months back and got rectified (the coolineg system was serviced including the fan), this much overconsumpution was not noted till recent days. The coolant is not reducing in qty. The oil is not giving any abnormal changes noted in the earlier days after changing it.
About the return pipe, I am not sure, but technicians were discussing something about that also, which I have no idea what they meant.
Kindly comment further if you find some thing more based on the above info above. Thanks