Difference in 10-30 vs 10-40 oil weights?

Answer 1

When you see a W on a viscosity rating it means that this oil viscosity has been tested at a Colder temperature. The numbers without the W are all tested at 210° F which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210°. The difference is when the viscosity is tested at a much colder temperature. For example, a 10W-30 motor oil performs like a SAE 10 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear. A 10w40 oil has a higher weight at 210* than a 10w30 oil. Obviously the use of a 10w40 in a modern engine would be foolish. Modern engines are built to much closer tolerances than engines of a decade ago. 10w40 does not flow as well at 210*, this causing engine wear.