A. Oils listed with W (which stands for winter) can have different numerical values different combinations within what is called a multi-grade diesel engine oil. For example, viscosity grades in the market today are 15W-40, 10W-40, 10W-30, 5W-40 and 5W-30. The lower the number next to the W indicates the oil can flow more easily at colder temperatures and can be pumped more easily into the engine. You should always refer to your engine manufacturer’s temperature application range for the correct range of temperature for the various viscosity grades. For comparison, an engine manufacturer may rate a 15W-40 oil only down to +15F whereas as oil such as a 5W-40 will be rated by the same engine manufacturer down to -20F or lower. Ultimately the advantage of the lower W- viscosity grades in cold weather can significantly offer several start up benefits including reduced engine wear, improved fuel economy, less stress on batteries and starters and less dependency on electrical block heaters which may also use less electricity for a fleet in the winter. As an added comment, cold box testing comparing 15W-40 oil to a 5W-40 resulted in the 15W-40 taking 5 times longer at 32F to reach the engine parts. However, the other factor is high temperature and many people think the 5W-40 is too thin at higher temperature but the 5W-40 is actually rated at the same high temperature. The thing that is often difficult to understand is that a lower W number improves flow at start up but at operating temperature achieves the viscosity range of the higher number providing the necessary oil film to protect engine parts. There are other factors to consider with multi-grade oils but this is the basic concept to consider.