I may be missing something here, by knowing nothing about lubricating oil.
Gallons are volume, so can be converted directly to cubic feet ... even if they're empty.
There doesn't need to be a substance involved, and if there is, its weight is irrelevant.
42 US gallons ===> 5.6146 cubic feet (rounded)
To find the number of gallons in the barrel, divide the total weight of the water in the barrel by the weight of one gallon of water: 381.8 pounds / 8.3 pounds/gallon = 46 gallons.
Approximately 6.073 pounds per US gallon.
1 US gallon of water is approximately 8.35 pounds. -1 Imperial gallon is approximately 10 pounds.
One US gallon of gasoline is approximately 6.3 pounds
A bushel of cracked corn is 56 lbs, which is about 10 gallons (or 2- 5 gallon buckets). So a 55 gallon barrel can hold approx. 5.5 bushels which approx. 308 lbs.
The weight of a gallon of benzene is approximately 7.01 pounds.
A gallon of water weighs approximately 8.34 pounds.
Approximately 6.5 pounds
A half gallon of water weighs approximately 4 pounds.
Approximately 6.6 pounds of acetonitrile can be found in 1 gallon.
An empty 55-gallon steel barrel typically weighs around 30-35 pounds.
Pounds of what? Feathers, lead, milk . . . .?