What type of oil is typically recommended for most diesel engines?

Multi-viscosity oil is typically recommended for most diesel engines because it provides better engine protection across a range of temperatures and operating conditions. The “15W-40” or “10W-30” designations indicate that the oil can perform effectively in both cold and hot environments, ensuring optimal lubrication during engine start-up in low temperatures and maintaining viscosity at higher operating temperatures. This type of oil is designed to flow well when cold (the first number before the "W," which stands for winter) and to remain thick enough to protect engine components as temperatures rise (the second number).

Using multi-viscosity oil helps to enhance fuel efficiency while reducing wear and tear on the engine, making it suitable for the varying conditions under which diesel engines often operate. Manufacturers often specify certain viscosity grades to match specific engine designs and operational requirements, further emphasizing the importance of adhering to these recommendations.

The other types of oil mentioned, such as straight viscosity oil, Pennzoil synthetic, and non-detergent oil, are generally either not suitable for the wide-ranging conditions that diesel engines face or may not provide the necessary protection and performance that modern diesel engines require. Thus, multi-viscosity oil stands out as the preferred choice for diesel engine applications.