Ibvex,
No. 6 oil Btu content - depending on how accurate you need it to be - the best way is to have a fuel analysis done and then use Perry's Chemical Handbook to calculate BTU's (if the analysis did not state it) or from Handbooks :
No.6 Oil Gross Heating value
18,126 Btu/Lb
153,120 Btu/Gal