Question: what is the difference in weight of motor oil?
Answer: The viscosity of hot oil is measured using different test parameters than when the oil is cold, so the numbers after the “W” don't relate to the numbers in front of the “W”. The difference between 10W-30 and a 10W-40 is the high temperature viscosity. Obviously, a 10W-40 is thicker than a 10W-30 at high temperature.