Diesel engines require a high viscosity oil that is able to withstand a significant amount of force. Most diesel engines suggest using 15W-40, as a result of having a heavier and thicker consistency. Diesel engines undergo combustion at a higher compression ratio, which requires a better performing type of oil.
Diesels engines, particularly newer diesel engines, also use synthetic oils such as 5W-40. This type of oil sometimes provides better results when compared to 15W-40 oil. Although synthetic oils cost more than other types of oil, they have the advantage of offering increased protection against wear and tear on the engine.