Perry VI, chapter 9 refers, among others, to liquid fuels and there you may find all relevant typical physico-chemical and combustion properties of No. 6 fuel.
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
No 6 Bunker oil, what is the specific gravity of the oil you intend to use as the heating value differs from 18,790 BTU/LB to 18,540 BTU/LB and the Deg API is from 15 to 10.
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For a heavy fuel having a low calorific value of 9750 kcal/kg the amount of theoretical air would be 10.8 normal m3/kg. As a ROT, for every 1000 kcal/kg of low calorific value one needs about 1.1 normal m3 of air and ~ 1.2 normal cubic meters of flue gases would be produced. But, of course, depending on the particular heater design, one may need to use excess air on top of these rather theoretical estimations.