Many people use the term density of motor oil to refer to its viscosity. As motor oils are rated by viscosity, it is typically the more useful term.
Density is a physical property of matter and is unique for each element and substance. It's a measure of the relative "heaviness" of objects, each with the same volume. The imperial units of measurement for density are pounds (lb) per cubic foot.
The density of motor oil is 54.9 lb/cubic foot.
Viscosity is a measure of how fluid a liquid is and varies with temperature and pressure. Motor oils are rated by viscosity and the numbers on a can of oil refer to this. For 10W30 oil, for example, 10W refers to the low temperature behavior of the oil when starting the car and 30 refers to the behavior of the oil at high temperatures after the car has been running for a while. Oils with low numbers are generally less viscous, or runnier, than oils with high numbers. The lower the first number, the easier the engine will turn over when starting in cold climates.
