How to determine how many gallons of oil your oval tank holds?
Your tank is in fact not oval, but stadium in profile. (Yes, that's where the sports facilities got their name.)

In the case of a tank measuring 60" &times; 40" &times; 27", your volume is (nominally) that of a box measuring 60 &times; (40-27) &times; 27 plus a cylinder of diameter 27 (radius 13.5) and length 60. 

60 &times; 13 &times; 27 = 21,060 cubic inches (91.17 gallons)

&pi; &times; 13.52 &times; 60 = 34,353 cubic inches (148.71 gallons)

Add those together and you get 239.88 gallons. 240 gallons is a reasonably common size for a residential oil tank.