Which of the following engine lubricants must be used if an engine is equipped with a DPF?

Using API CJ-4 engine lubricant is essential for engines equipped with a Diesel Particulate Filter (DPF) because it is formulated to meet the greater demands of emissions control technologies. This lubricant is specifically designed to work in conjunction with DPFs and other after-treatment devices, which help reduce harmful emissions. API CJ-4 provides better protection against soot, higher temperatures, and oxidative stability, all of which are crucial for maintaining the efficiency and functionality of engines that must meet stringent environmental regulations.

Additionally, API CJ-4 contains lower levels of sulfated ash, phosphorus, and sulfur, which helps to extend the life of the DPF by minimizing the buildup of ash that can clog the filter. The formulation is tailored to ensure compatibility with exhaust after-treatment systems, thus maintaining the engine's performance while also protecting the environment.

In contrast, alternatives like other API classifications, particularly those predating CJ-4, may not offer the same level of protection or compatibility with DPF technology, potentially leading to engine and filter issues over time.