Medical physics Question:

As a rough guide engine oil usually weighs about 0.87 kg/liter or about 7.2 pounds per US gallon.

Unfortunately a rough answer is the best that can be done as there are hundreds of different types of engine oil with slightly different 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 varies with temperature.

Most oil manufacturers' provide data sheets for their products that should include density. If you search the manufacturer's web site you should be able to get an accurate figure for a particular grade.