I also read today that the MKII Lancaster had a lower service ceiling and a lower maximum bomb load (possibly because of the fuel you mention)Hercules was heavier, bulkier and consumed more fuel, the Merlin versions of the Lancaster were better climbers, slightly faster and had more range.
The Lancaster datacards show for the Hercules VI equipped version 2150 gals fuel load, 395 gals allowance and 2070 miles range with 6k lbs of bombs (Lancaster II with 65k lbs max weight)
Lancaster I/III with 68k max weight, the same fuel load has 270 gals allowance and 2250 miles range with 10k lbs bombload
difference in climb to reach 20k ft is 4 mins in favor of the Merlin versions, 41.5 vs 45.5 mins