Used to tank up aircraft for ferry flights in the 1980s.
IIRC the BN2 Islander could have 4 x 45 gallon/205 Ltr drums in the cabin giving it a duration of 10 to 12 hours. Used to put 6 drums in the Trislander.
Did a Twin Otter with 10 drums to go to New Zealand. It's first sector was Bournemouth to Cairo, about 14 hours.
We even got 3 drums in an Aztec. and 4 in a Navajo.