LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

15W-40 Oil in Diesels

SAE 15W-40 has traditionally been almost universally used in heavy-duty (HD) diesels for a long time. The reason is that, generally, thick, viscous oil compared to thinner, less viscous oil provides a good hydrodynamic wedge and better load-bearing characteristics at normal engine operating temperatures. The use of 15W-40 with a hot viscosity of SAE 40 in HD diesels is needed to support heavy bearing loads, a characteristic of HD, high power output diesels operating with very high cylinder pressures. 15W-40 consists of a base oil of SAE 15W that has a viscosity of SAE 40 at operating temperature.

The main advantage of multigrade oil is improved cold-start characteristics with less engine drag. At operating temperature, a 5W-40 oil and a 15W-40 oil will have the same viscosity. When cold, the thinner 5W oil will have less cranking resistance, but it may not provide the protection to bearing surfaces that a 15W oil can. Many HD diesel engines today are capable of operating with 10W-30 viscosity, whereas 15W-40 was mainly used up until 2010. Now, other grades and classifications of oil are replacing 15W-40 to reduce fuel consumption and to meet GHG targets. Enabling the use of thinner, lower-viscosity oil are newer specialized anti-wear additives that can prevent metal-to-metal contact and new bearing technology with harder shell surfaces and scuff-resistant coatings.