The oldest oil refinery in Asia is located in:

The oldest oil refinery in Asia is located in Digboi, Assam, India. It was established in 1899 by the Assam Oil Company, a British company. The refinery was originally built to process crude oil from the Digboi oil fields, which were discovered in 1889. The refinery has a capacity of 3.8 million tonnes per annum and produces a variety of products, including gasoline, diesel, kerosene, and fuel oil.

The other options are incorrect because they are not locations of oil refineries. Guwahati is a city in Assam, India. Bongaigaon is a town in Assam, India. Numaligarh is a town in Assam, India.