In addition to Bally H's observations, I must add that the fuel figures quoted seem to be rather odd. Having flown 767s from the earliest -200s registered in Europe to the latest -300s, all models used versions and developments of the CF6-80 (other operators use RBs & PWs). The CF6-50 (which I assume you meant, Guv) was fitted to DC10s which are not involved here, and hence are not up for comparison.
Each development of the CF6-80 that I have flown has resulted in an increase in thrust and a reduction in specific fuel consumption.
As fuel has become the largest DOC in the past couple of years, the following may be of interest (with thanks to dispatch).
This morning, L1011-200 LGW-YQX - 360 pax - TET: 4:55 - FBO: 44 k Kg (97 k Lb) = 8950 Kg/hr, or 24.8 Kg/pax/hr. If flown by 767-300, same route, (328 pax), TET: 5:05 - FBO: 25.5 k Kg (56.1 k Lb) = 5020 Kg/hr, or 15.3 Kg/pax/hr..
Now, whichever model we're talking about, that puts the L1011 fuel/pax/hr hugely (60%+) above that of the 767. Even with the additional 32 pax, that's an awful lot of maintenance and spares before you draw even! Where the 20% "less" comes from, I simply cannot see. [Figures for the A330, anyone? - The 777 has yet to enter the IT Charter market, so is not really relevant]
As a fellow Gatbasher I nonetheless continue to wish you well in your endeavours, Guv.