Question: what are the differences in oil weight?
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.