The weight of a gallon of oil depends largely on the oil type. The exact weight can also vary based on what temperature the oil is stored at. Most common oils stand between 7 and 8 pounds, while stored crude oil is around 7.2 pounds per gallon.
Recycled motor oil weighs 7 pounds per gallon, crude oil is around 7.2 pounds per gallon, and olive oil is around 7.6 pounds per gallon. Heavy oils like glycerin can be up to 10.5 pounds per gallon.
The reason the weights of oils tend to fluctuate is because temperature affects the viscosity and density of liquids. Most oils become more viscous and less dense as the temperature increases.