How much does automotive oil weigh?
As rough guide engine oil generally weighs about 0.87 kg/litre or about 7.2 pounds per US gallon unfortunately a rough answer is the best that can be completed as there are hundreds of dissimilar types of engine oil with slightly dissimilar densities. To get an exact density we would have to know the details of the oil you're using. For a really accurate answer we would also have to state the temperature as the density of oil as with most fluids differ with temperature.
Most oil manufacturers' give data sheets for their products that should contain density. If you search the manufacturer's web site you should be able to obtain an accurate figure for a particular grade.