A Gallon of oil will weigh just a bit less than a gallon of water.
The weight of a gallon of oil is around 7.2 lbs while a gallon of water weighs around 8.36 lbs depending on the density of the water and the surrounding temperature.
The temperature can make water weight more or less depending on the temperature.
But the gallon of oil will weigh less than a gallon of water.